Background: We have teenage children of our own and we are also guardians for a 17 year old "Ben" (not real name obviously)
Ben has been going out with a girl for the last 12 ish months or so (t's been a bit on and off) but they have known each other for a bit longer than this as were good friends beforehand.
His girlfriend lives with her mum and step dad and they are getting married in January. After the wedding they are planning to go to Mauritius for 2 weeks and Ben's girlfriend will be joining her parents. The issue is that Ben had been invited too.
To begin with, we said no because as much as would have liked him to go, it was a hell of a lot of money. It's not that we couldn't have found the money because that would have been possible but it was not the sort of money we have lying around.
Plus - relationships can be tricky at the best of times (particularly when you're younger) and there was obviously a risk that the relationship would break down prior to the holiday and it would be doubtful we'd get our money back.
However the girlfriends parents came back and told us not to worry because they had intended to pay for everything themselves (his flight, all inclusive package etc) but suggested he should bring some spending money (no problem)
They made it sound as if they really wanted their daughter to have him there as company so that they could enjoy their honeymoon shag-fest
So we agreed and obviously thanked them profusely etc etc as it was an incredibly generous offer.
Well now you can probably guess what has happened - they've split up and things are hideous between them. Nothing amicable what so ever and they are no longer even friends. He's devastated 
His ex-girlfriend now wants to invite someone else on holiday with her (not sure if just friend or new boyfriend) and that is obviously fine and non of our business.
But her parents now want to be refunded for the money that they spent for Ben and want us to pay them £2100.00
There is no fucking way we are prepared to do this as the agreement all along was that they would cover all the costs (on their insistence) but they are saying that we have no proof that they had insisted on paying and so we should cough up.
DH's point is that neither have they any evidence that we had agreed to pay so they can get stuffed.
They have now passed 'messages' to Ben at college (via his ex girlfriend) to say that we had agreed to pay and have now changed our minds and that he needs to speak to us and ensure we pay them.
He is now incredibly stressed about it and seems to be believing their version of events and also thinks we should pay.
I've tried so hard to explain to him what has happened but he is just too upset and embarrassed about the whole thing and wants it to go away
He has approx £1500 in an ISA and has said that if we don't pay, he will use that cash and I just want to fucking cry.
This is a boy who has been to hell and back over the last 10 years, I can't even go into the details of what he has been through but 2 years ago I got onto a bus and found him sitting at the back soaked to the skin as he had been wondering about in the rain and sleeping rough for a few nights with the clothes on his back. He had been completely disowned by his parents (complete and utter fucking cutting bastards) and long story short - he ended up living with us.
We managed to support him through his GCSE's and he scraped enough grades to get into college. He has been doing so well but is pretty fragile and now this has happened.
I can't and won't pay this bloody money (DH is now facing a possible redundancy which complicates things and is a massive worry) so I have no problem what so ever 'sticking to my guns' as far as they are concerned.
But what about Ben? I feel as if by refusing to pay I will be destroying everything we have worked so hard to achieve on his behalf (our relationship for one thing as his relationships with most people are quite fragile)
I'm not a push over. I'm a very upfront, confident and outspoken person. I would never allow myself to be blackmailed or intimidated by anyone. I am too old for this shit but I feel utterly stumped.
I literally don't know what to say or think and I'm so angry. I need someone to talk me through this as I'm terrified of ruining everything that this amazing young person has achieved.
We should never have agreed to the holiday but then we couldn't really refuse. He's not our child, we have no legal guardianship for him, he could have just gone anyway if he had wanted to.
I am pissed. As in actually quite drunk. And also pissed.
I'm not even going to spell-check this and can't be arsed with smiley's either. I just want to howl