I wouldn’t have the same expectation of privacy using a library computer and printer as I would at home. I read other people’s print jobs and faxes at the printer at work; if you want it to stay private in a public place, it’s up to YOU to make that happen. My eyes don’t stop working just because you’re printing private stuff at work.
“I couldn’t help but notice…”
“By the way, many patrons lately have been having trouble with Nigerian get-rich quick schemes, and I’m trying to warn everyone who uses the library’s computers that…”
"I read about this guy on the dope who hooked a Nigerian scam artist and made him convert to a new religion and send photos of himself with red paint all over his body. "
Whatever it takes. He needs to know. You don’t have to admit you ever read anything.
In other words, you would just make something up that wasn’t true?
I really don’t understand the whatever it takes solution to a question about professional ethics. What kind of moral code suggests lying rather than the simple and straight-forward truth?
The OP did not do anything wrong. She did not deliberately read his material. But once she noticed that the material is a scam, it is reasonable that she tell him that she noticed.
He may accuse her of reading his material. He would be wrong. He may be angry. His anger will not harm her. He may ignore her warning. That is his business. On the other hand, he may heed her warning and appreciate her assistance. That would be most rewarding. She just can’t choose what will happen. She can only choose what is the right course for her. Certainly the right course is not just making up a lie to make it easier to “do the right thing.”
I think it’s a good think that she tell her supervisor first. In fact, it would be good if her supervisor could accompany her when she tells the gentleman. They can both show concern.
Absolutely I would tell him - I would phrase it “Excuse me sir - I couldn’t help noticing…” - which is actually truthful as well as helpful. I would print out this page and urge the patron to read it carefully.
Couching it in general terms (I’ve had patrons asking about this) is a way of softening the blow, so to speak. This guy is dim enough to fall for this; he’s dim enough to think he’s the ONLY one this has happened to. I’m sure he’s not the only patron at the library, ever, to have had concerns about financial schemes. What is the big deal about “lying”? You’re not under oath; you’re trying to help a patron.
What is preferable–to tell him he’s dim (but it’s The Truth! what’s that? you wouldn’t say that to someone because it might hurt their feelings? Which side are you on, senator?), he’s fallen for a con man who lives on another continent and that the library staff all know, thereby increasing his humiliation and embarrassment?
have at it. How is giving the guy more info a Bad Thing? It sounds to me like you’re afraid to be “yelled at”, which makes me wonder how long you’ve had this job and how old you are. You cannot control his reaction. Are you going to feel better if you say nothing at all? Kicking it upstairs to your department head/boss/supervisor is just passing the buck. Better to talk it over with your superior, and (hopefully) try to inform this guy. If he doesn’t listen, so be it. You did the best you could. Anything less is a disservice to your patron, IMO.
I would tell him. I would start with, “it’s none of my business, but…” Of course, I’m not a librarian, but that is what I would do.
Simple.
Post a short memo about scams, including the Nigerian scam, **on all of the computers in the library **with links to scopes.com and suggest they speak with the librarian if they have any questions “about people trying to scam you.”
BTW, I teach at a college and even students there fall prey to idiotic scams like, “Bill Gates will send you $200 if you forward this email”…etc. They have never heard of scopes.com. I really need to do a class on this subject, so thank you for reminding me to do so.
Neither have I. 
Ahem…it is a famous site about the Scopes trial. Not, of course, to be confused, or typed incorrectly with www.snopes.com that is probably a tad more relevant to this thread.
I forgot to update - I did tell him. (Found out later that so had one of my coworkers.) He got furious at me, stormed off, threw out the materials I’d printed up for him. Hey, I did my best. He hasn’t asked me for any help since, though, which is kind of sad - on his end, I’m sure, I broke his trust. He probably thought I meant to steal his Nigerian millions.
You did your best. Hopefully he’ll come back and thank you later, when he researches it a bit more and discovers how close he was to financial ruin. But whatever happens - you did the right thing.
If he’s too dim to get it, he’s too dim to get it - you can’t save everyone. You can sleep with a clear conscience that you did what you could for him.
Add me to the growing list of posters who think this is the way to go. If he ignores you or gets mad, just smile and let the Nigerians have all his money. You’ll have done the right thing.
He’s probably sent them an ass-ton of money - he’s here all the time obsessively checking his e-mail. He avoids me.
See? win-win! 
There are some people (probably many) that just don’t want to be helped. At least you tried. Well done.
Damn, girl. I’m sorry to hear that. You did the right thing and tried to help.
Sometimes you just can’t win.
Perhaps you could mention in passing, for his own edification, that [most] libraries log user information. …And, what’s a Nigerian scam?
You receive an invitation to launder, invest, win or inherit a large sum of money - after they’ve hooked you, they tell you you have to pay some taxes, transfer fees, documentation cover charges, etc - you pay this money, then they try to milk you for more, in the hope that you’ll throw good money after bad. Obviously you don’t ever receive the milions of dollars, because it never existed.
In some cases, the scammers will invite the victim to fly out to meet them (bringing expensive gifts for the lawyers, more cash, etc - if this happens, at best, the victim will be mugged and left without anything (including passport), at worst, the victim will be held to ransom, or murdered.
I think you should take a picture of him, and tell him it’s for all your friends on the internet who want to see what someone who would actually fall for that scam looks like.
Cheese!