Your looking at a 50-50 chance. You need to have her assessed by a vet. None of us here can tell you if she's bred or not. But it's likely. A 17 month old colt can definitely get the deed done, and since it's spring, your filly very well could have been in season. If she is bred, the vet can lute her and terminate the pregnancy, but there is a window of time for it to be done in so it's best to know sooner rather than later.
As for them paying for the bill, well, accidents do happen. Especially with horses. If they were supposed to be in charge of making sure that your filly was safe and settled in, then I'd want them to pay for it. But I think that it could turn out to be very ugly if the owners don't see things from your point of view. I know that's not very helpful, but I can see both sides. They may try to argue that YOUR filly just in with the colt...Not the COLT jumping in with the filly, therefore it's your horse that caused the problems, therefore it's your bill.
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."