I think it's up to the bowyer to examine the veneers, and let you know what he thinks. Frankly, if he had ordered the veneers himself, he would be in the same place, and he would have to decide whether they were worth working with or should be sent back. If he decided to work with them, he should still have a clause in his agreement that states that he is not responsible for any damage caused by use of customer provided wood. I would imagine that many bowyers will not think to have this clause until they are stung. In the absence of an agreement in advance, the bowyer and the customer would have to negotiate if there were problems with the wood, never a pleasant situation.
I have never provided my own wood, but I have had several bowyers tell me that they needed more time because their own wood didn't work out for some reason or another and they had to remake part or all of the bow as a result. So it's not that unusual of a thing to happen, but of course, if it's their wood, it's their dime, although it's your time. (Probably better stop before I start trying to rhyme all the time!).