Agreeing with everyone saying that there are a number if ways this might have come about, not all of which paint's the girlfriend's mother in a bad light. However, i would strongly encourage to to work it through, rather than just say a blanket no. A blanket no will focus all his anger and resentment on you, very fairly and give him much less chance of learning what a bad idea all this is...
At a similar age, my parents always just said no, no discussion. My response was to sulk a bit for a while, well until i had worked out how to do something much more dangerous, with much more shady people, which they never ever found out about. Think hitchhiking, staying overnight well, where ever we happened to be. Thinking back, well, i'd rather not.... But they thought they were keeping me under control and now I get that they loved me and only wanted the best for me. At the time, they were just an obstacle to be overcome and wow, i got good at that.
I really would talk to the parents first, quite i formally, asking them their understanding and work from there to a position you can agree about. The statement money not needed til later sounds as if they are buying the tickets and only expecting to be paid back later....If its true, maybe they are wanting their daughter to have company on the holiday and are keeping their options open and prepared to pay the admin fee to change the name if necessary... They should be aware that you are not in a position to pay, and why should DS1 be paid for at he expense of DS2 ? and make sure you both are aware of what is being agreed.
Then enjoy....all the best.