Neutering a male with a retained testicle is similar to a spay for a girl because they have to go into the abdomen. Your boys will be no worse off than if they were girls and got spayed. Undescended testicles put the dog at a thirteen times greater rate of developing testicular cancer. I would go with what your vet says on this. They've seen it before and will see it again.
I sympathize with your girlfriend. All surgery has risks. The best way to try and eliminate those risks is by doing preop bloodwork, using an IV catheter, doing surgery on a heated table and using the correct kind of anesthesia.