Slms. Firstly, abortion should not even cross ur friend's mind. This child is innocent and a gift from Allah(SWT). Right now it may seem as if this pregnancy is a punishment or a burden, but Allah only knows the joy n love this child will bring into ur friend's life.
I suggest that she tell her husband that she is pregnant. He already knows that she had an affair and surely ur friend and her husband as well as the man she had an affair with, know that by having unprotected sex u stand a chance of falling pregnant. I'm sure her husband will be angry, maybe their marriage won't even survive this kind of a scenario, but maybe he will find it in his heart to forgive her n raise the child as his own. Either way, this is the test that Allah has given to ur friend and she now has to make sure she seeks Allah's guidance on how to survive this test.