Ultimately you’re posing a question to the TM about how to proceed from here and I think there’s a consensus forming around the thought that an obsession with revenge is a cancer that holds little hope of satisfaction for your friend, and beholds a great deal of potential damage to both your friend and innocent third parties, and it’s best to just move on.
Things that have made messes in my laundry:
- Gum
- Ball point pens.
- forgotten tubes of model airplane cement in pockets.
- New fluffy towels that clog the lint filters.
Let’s see…
- “chicken cutlet” pads
- rubber swim caps.
- spray bottles of perfume that broke in the dryer.
I used to share a house with a carpenter (Builder type, not the singer) and he was banned from all of the local laundromats because he often left nails in his pockets when he put clothes in the washers. The nails would find their way into the holes in the drum and tear the lining!
Personally, I don’t understand the point of revenge. You do something stupid that brings you to another person’s stupid level… whoop-de-doo…
My preferred approach is to go out with a group of friends, have a few drinks and munchies, talk all kinds of trash about the worthless bag of protoplasm, plot exquisite and exotic tortures, then get on with life. Revenge is stupid…
screw messing with his machines. Corrode his brake lines so the burst the next time he has to mash on them. Anyone worth taking revenge on is worth taking serious revenge on, anything less is pansy assed b/s. If you are dead set on messing with his business don’t do it in a way that ruins someone else’s clothing or life(ie: colored sneakers in a dryer or amonium-chloride gas), get into his mailbox and swipe his insurance payments before they go out. Then keep checking back and swipe his notifications of a lapse and termination of coverage notice and then firbomb the place after his policy lapses.
Of course I am an extremist so this advice may not be suitable for you or possibly illegal in your state. 
Thanks to everyone for their ideas. Rest assured, she is not going to do anything to other people’s laundry. The idea is to throw some old blankets and towels into a machine along with the offending stuff, start it, leave, and have an unknown person (me) go in about 1/2 hour later to see the results.
Mashed potatoes, dye, pizza, and nails sound good. Also a few boxes of Kleenex in a machine.
I still don’t like it. Maybe, despite your alerting him, he won’t put an Out of Order sign on soon enough and it will be the next customer who finds it. Imagine some tottering little old lady who is–alone–raising her two grandchildren who generate tons of laundry. She puts in a load of clothes and diapers, her eyesight too bad to see the kleenex on everything, or the dye. She puts in her quarters which represent a disconcerting portion of her disposable income, and pours in the last of her detergent. Only then does she discover the machine is compromised. Now imagine that little old lady struggling to lift the sopping clothes out of the soapy dirty water to get them to a new machine. Imagine her crying when she sees her granddaughter’s only good dress covered in dye or covered in kleenex lint that will take hours to pick off. Imagine her pleading for a refund of her quarters… I realize I’m being melodramatic. but this is what happens when you try to take revenge. Someone else–someone innocent–is often harmed by it.
Or imagine the boyfriend does find the damage before grandma does, and he assumes it was a regular patron or an unknown prankster. So he initiates new rules and policies. He ups prices to cover the damage. He screams at the attendant who was on duty when it happens. In short, he does stuff to make life miserable for other innocent people. It’s not such a happy outcome, and it’s not justice.
Take the money you were going to spend on kleenex, nails, and dye, and send $5 cash donations to some of the most obnoxious and persistent evangelical agencies you can imagine, in his name. Let him try to get off their list after they have him listed as a bona fide donor. Or better yet, take the high road and give that money to a legitimate charity. Your friend can feel the extreme satisfaction of knowing that she was way above his level, and took a potential sow’s ear and made a silk purse out of it.
Believe me, someone who is nasty and evil and unfair (I’m talking about the boyfriend) will eventually get what is coming to them. Maybe you won’t get to see it, but trust me, they eventually reap what they sow.
Often women who want revenge against an old boyfriend would trade the revenge for a kiss & hug from him.