Sounds awful but a friend of mine had bought tickets for a band later this month and I really don't want to go. DD is waiting for a surgery date, fairly stressed, I said once already that I can't afford the associated costs, 60 miles away, then she said that we could stay at a relative of hers so a hotel wasn't needed. Feel railroaded! What to do?
Unfortunately I think you left the door open for this by not being honest that you just don't want to go. You said the problem was accommodation costs so she solved that.
Do you not want to go full stop, or do you not want to commit to going in case DD's surgery date coincides? If the latter, I would explain the reason and stress you may need to pull out at short notice. If you really don't want to go I think you should say "I'm sorry I wasn't up front about it but I don't like band [x] and I don't want to go".
Well, it was her risk when she bought them as a 'surprise' - would you want to go if it weren't for the operation? Presumably as well as the overnight costs for you you'd also have to find someone to babysit your DD? I wouldn't want to go in your shoes, I think I'd just say it's too much to take on when you've got the operation coming up but that you hope she and the other friend can find a third to take the last ticket (no skin off your nose either way, however - you never committed to going to this thing in the first place).