I was diagnosed with gallstones in early November after a number of attacks and generally feeling crap, my GP phoned to confirm on the Thursday (my birthday, ha!), I started feeling really ill on the Saturday and by Sunday night I was in the Emergency Surgery Unit via A&E with acute cholecystitis. Was in until the Friday and had a miserable time of things up them them taking it out on 9 March - had a laprosopic job and out in a day but it was on the way to septic and had split with stones having escaped - I had gone yellow before the op and had liver stuff even showing up in my wee on tests. I was off work for 2 weeks and even then it was hard going so I wouldn't have considered going abroad for at least another 2/3 weeks after that (I had underestimated recovery).
Going to a festival, let alone abroad, would not have been an option for me. I hardly went out save work and day to day stuff let alone ate out. I stuck to a low fat diet and still suffered, as Jack said it can help not aggravate things in some cases but won't stop attacks or worse. I'd have felt very uncomfortable with the thought of going and about anyone who tried to pressure me into going, especially if my health was at stake.
Even having general discomfort on a plane, let alone an attack or worse would have played on me, I wouldn't have enjoyed it. Worst case scenario, what it something went wrong in the air - I've seen in some airline conditions about charging passengers if emergency care or landings have to be undertaken, especially with reasonable forseeable problems (not sure how right that is).
Others have covered what it could cost in the states apart from having to go through that many miles from home with the person who persuaded you to go against your better judgment (I don't mean to sound too mean but I think that's how I'd feel).
Also note that, as I found out, they won't, if they can avoid it, operate on an infected gallbladder so it a stone got stuck and it got inflamed and infected then he could be in hospital a while before it settles enough for them to be happy to operate, then waiting for a surgery space and recovery (possibly longer if problems before) then the time before is allowed and is able to fly home (also taking into account the airline's ok to take him to fly). Factor in not just the treatment but the additional costs of accommodation and meals and other things, including likely having to get other flights back. It could end up costing you a hell of a lot more than cancelling.
I hadn't realised how bad gallstones could be before, how ill they can make you and, especially once you've had problems once, the fear of it happening again or something going wrong (things can get very serious and very quickly!).
Having been through it, there is no way I'd go.