It sounds like she needs to stop falling into his emotional trap and arguing with him. Instead, bring it into the realm of thoughtful decisoion making.
RIC is wrong morally and philosophically. If someone chooses to advocate for RIC, they must be the one to provide arguments why the advantages outweigh the moral and philosophical reasons against it. They must provide the references and you must be able to follow the logic. If not, then realize you are agreeing to emotional and irrational arguments. We do that all the time, so nothing says that is wrong...however, it should not be done lightly, especially when you are supposed to be playing an advocacy role for your child to protect him from harm.
Bottom line is the default should be remaining normal (intact). Any other course needs strong arguments to justify it. Not strong emotional outbursts, not strong irrational feelings, but strong arguments that show the advantages for the child, supported by good evidence and a logic that others would agree is compelling.
The core issue is that it isn't her husband's penis. It's thier son's penis. And their son is the person who has the right to decide how he wants his penis to look, function and feel.
The risk of complications is why many men and women in the USA now work to end the practice of circumcision. When it is done on an infant and there is damage to the sexual functioning of the penis, it is not known until many years later. The victim must live with a decision others made for him when he was a baby. When an adult man takes the risk, he must live with what happens, because it was his own choice. An infant or child must not be exposed to risks that can cause a sexual disability he must live with when he becomes a man. Just because it worked out well for her husband to be circumcised (assuming he would say it did) does not mean that it will for their child.
Ask him how rational it is to have a cosmetic surgery on a non-consenting minor for the sole purpose of wanting his child's sex organs to look like his? Are they going to compare them? Is he willing to shave until his son grows hair? They're NOT going to look alike even when they're adults.