
I don't know about your breed, but I brought our WSS puppy home with two testicles descended at 12 weeks--and then took him to a show at 26 weeks, by which time he only had one! It came as a bit of a shock all round--his breeder was there at the show and I discussed it than and there with her. I didn't ask for nor did she offer any refund when he stayed the same. It would never have occurred to me that she should--this is just the way it turns out sometimes. I wouldn't swap our lad for anything or anyone in the world, and when he reached 14 months I booked him in and paid for the retained testicle to be removed. He'll never have pups, but there could never be another like him anyway.
Your situation is a bit different since your boys are going to new owners without their testicles in place. They are still very young and everything could still turn out fine. If you keep up your contact with both owners you can discuss appropriate options as and when--if she is keeping the pup "to see how he turns out", she'd be a rare owner to give him back! The more important issue is heredity and not breeding from that line is something you should consider.