I understand Sarah's actions, no matter if it was her place or not. If people are calling you a homewrecker for years and then start insulting your baby, I would have snapped, too. I don't have children, but touch my baby and I'll come for you. What the fork is wrong with people. Who would say such things to an innocent baby ffs? I'm surprised, more than anything, that she lasted this long.
edit; to add, even if she did it to protect herself, she has to think of the future of their baby too. if other kids some day find out it's possible they will start calling her mom names, you really don't want your child to get bullied for something their mom did not do.