Funny that you say that, that was the quip one of my coworkers made when she said she didn’t know most of the corridor route.
I agree with the other people who said that a letter is not a “light touch” at all, but if you can deliver it right, saying something like “we’re sorry to be giving you more to worry about at such a bad time, but there’s something we thought you should know about” (frankly, he should have noticed himself) then it can be. I’m guessing that you thought it would be better than all of you piling up in his office.
From what I know of labor situations in the US, you guys aren’t likely to have someone who’s a sort of “official delegate” for the rest, but perhaps the most senior person would be amenable to act as one? Either delivering the letter or the complaints.
Foxy40, I’ve faced complaints from other coworkers. Sometimes, said coworkers were an ocean away. They were never identified but I was able to guess them correctly and explain my side every time (my bosses were always amazed that I knew who it was by being able to figure out where they were coming from). Not being a confrontational person, I prefer this approach to “ok, you two slug it out and let me know who’s won” - so long as I’m allowed to say my piece. The only times I have still been punished were times when I wasn’t allowed to defend myself.
To address point #1 in your last post, you really would be best to forget about “fairness” in a job situation. i would be surprised if many folks were unable to come up with numerous examples where people were treated differently in their workplace. Moreover, you do not know all of the info the supervisor has. Perhaps he (or your co-workers) believe you are receiving some concessions you do not perceive as such. In any event, don’t focus on whether your co-worker is getting more than she deserves. Focus instead on whether you are being mistreated.
Harriet gave some really good advice. Rather than “complaining” (a word you use often), inquire as to your duties and responsibilities, and future at that job. Provide examples and possible solutions. As far as you are concerned, your main concern should not be whether she is getting more than she deserves, but instead, whether you are getting as much as you want/deserve. IME complaints rarely reflect well on the complainer. But signs of initiative and desire occasionally do.
Also, bosses tend to respond to requests to benefit from their experience. Big difference between saying “Why did you give her more hours than me?” and asking, “Gee, I really would like to get more hours. I notice she is getting more. What can I do to get myself in such a position?”
As furl and others suggested, stop enabling this girl. Don’t say why. Don’t suggest you are disciplining her. Instead, just say you are busy. “Sorry, can’t stay late tonight.” And turn a blind eye to her oversights or needs for assistance (so long as that will not cause you trouble). Or you could propose that there be a system for documenting lapses. You can present it as a desire to improve performance and address a problem, without making it appear personal. (So long as you are comfortable that her misdeeds will show up more than yours!)
Once you get to a position where you are getting what you need/expect out of your job, you can come to really appreciate the low producers, deadwood, and fuckups among your co-workers, because they make you look good simply by comparison without requiring that you exert any unusual effort or doing anything special! 
Good advice, [d]Dinsdale**. One of the most effective ways I’ve found of letting a supervisor know about a bad co-worker or situation was to come to them and ask for their advice on helping me figure out what to do about it. For the examples you’ve given, Captain, you might go to your supervisor and say something like, “I’ve shown LB the corridor routes three times now, and she doesn’t seem to be getting it. Do you have any suggestions for how I can teach her this better?” It tells your supervisor everything he needs to know, without you looking like you’re complaining, and puts a positive spin on it - you’re trying to solve the problem, not just complain.
Foxy40, I had a supervisor who used the same method that you used when I was trying to deal with an abusive co-worker. The end results were:
- co-worker escalated her abuse
- I quit and company lost fabulous employee (me)
- I got employment insurance benefits because I had documented the abuse and my efforts to fix the situation
- company got investigated by Canada Employment agency for not protecting a worker from abuse
- company ended up firing abusive co-worker and needing to replace two workers