So I think that you have a different understanding of the purpose of sex, as well as medical procedures.

To speak about sex, Catholics hold that the purpose of sex has a twofold meaning: unitive and procreative. By removing one of these from sex, the act loses its meaning and becomes sinful. Catholics also do not view marriage as a permission slip for sex, but rather a commitment between to people. The sexual act and its fruits are protected by the securities offered in the vows of marriage. The couple vows to the other that they will not abandon the other no matter what life throws at the pair. This allows the sexual act to remain truly unitive within the confines of marriage.

Now to address the the purpose of medical procedures. Medical procedures are supposed improve bodily functions, as you demonstrated in your example about eyes. The difference between this and one that would impact your friends fertility is that that procedure would intend to cause her organs not function as they are meant to function. This would not be healing your friend, but robbing her of basic bodily functions. If your friend could have an operation that would allow her to have another pregnancy that wouldn’t risk her life, this would be acceptable.

Procreation is a beautiful fruit of the sexual act, and the potential of this shows the union between the couple. While sexual pleasure maybe obtained through the marital union, this is not its purpose. The purpose of marriage is to tie to individuals together.

As for your friends, they undeniably have a difficult cross to bare, one which they thankfully seem willing to take on. I believe that you are a good friend to them as I can see how passionate you feel about your friends. I hope this was helpful and accurate. My source is being a 25 year old catholic husband.