Remind them, maybe they forgot.
What do I do about a co-worker who has owed me $20 since February ?
It's been about 7 months. Let it go. I have a firm belief on money. If someone asks you to lend them $20 (or whatever), it works out much better if you just GIVE it to them. If you can afford to give it, then do so. It takes the stress out of the friendship as you are not constantly looking for them to repay it.
Most of the time when someone asks you for money is that they're broke and don't have two nickels to rub together.
If you have someone consistently wanting to borrow money, you need to set boundaries. You need to know if they are frivolously spending money on stuff so they can't afford the necessities....
So in 99% of the time when someone asks to borrow money, I just give it to them, if I have it to spare. I do have a friend that does not have any kind of clue on how to budget, doesn't have money for rent but has money for a new iPhone, etc. I deal with him differently than the friend that needs gas money to get to work. Hope this helps.
Remind them now, and again on pay day .. And on the payday after that .. And well .. You get my drift. .. OR better yet .. ask to borrow $20 and see where that goes.
Never lend money you can't afford to lose.