My best friend and I went on holiday in September. I'd never been abroad (hate flying) so it was a big deal. Was really glad she said she'd come with me and, as a result I said I would like to pay a bit of the costs (a bit of her holiday cost, the airport parking and a few other bits and bobs).
She earns a few thousand more than me but in other ways I am slightly better off (she rents, I own and my mortgage is less than her rent and she owes her parents some money for helping her buy a new car when hers died). I said she didn't need to pay me back immediately and meant it, especially as Xmas was on the way. She paid me back £100 in Sept, £200 in Oct and £125 in November. There's only £100 left.
Now, I meant what I said about no rush and I wouldn't normally mind if I didn't get the rest back next year, but two weeks ago she went away for a weekend with some friends which I know cost around £200 and she's just booked a ski holiday with friends costing £750 and is off shopping for holiday things today.
I just think if you can afford to do that, you can afford to pay your friend back and possibly ought to pay your debts before booking TWO more holidays. It comes across as taking the piss.