My sister has been dating a married man.

Hi there,My sister’s been dating a married man for 2yrs and a couple of months now,everything’s been going well between them.The other day she posted a photo of themselves on facebook,the wife’s cousin saw it & showed it to the wife.She called her husband & now things are bad.This guy & my sister are still talking but the wife is not picking up his calls,she (the wife)says she knows my sister by seeing her,she wants to come to our house & confront my sister and yet our family knows nothing about this.Poor girl is confused n doesn’t know what to do,a couple of people who know about this think she is a marriage wrecker.What can she do now?? part of her feels that she should apologies to the wife….Please help