Hi have struggeled to make this decition too. I was in a long relationship with someone and had 3 kids. The oldest one, wasn't from my ex partner (i'm not together with him any more). My oldest daughter, was from my first relationship and her father never much cared about her. In my second relationship gave birth to two more boys. As my daughter was 5 years, her biological father appeart after years and want to see his daughter. Now what, what should i tell my daughter who this stranger is. She was to this time 6 years old and i decided to tell her, that this stranger is her biological father. He wanted visitation and i let it up to my daughter if she want to or not. She decide to give it a shut. He visited her first every two weeks, then he started lieing to her. Suddenly he couldn't pick her up etc. After a while, my daughter was then 8 years old (he want to see her out of sudden again), she told him, in his faith, that she don't wanna ever see him again, because she don't like to be lied to. In all this years, she called my two boys father, dad and she still does ist. She know's what's going on and know's who was there for her the whole time.
What i try to say is, i think your friends child is old enough to understand. The kid will decide for her own, what's good for her/him. Sometimes you think kids don't understand and don't know what's the best for them, but sometimes we just think totally wrong. She should give it a try. Even if she should get devorce, i think her husband have enough godds to don't visit her afterwerts or treat her different.
I'm not together any more with the father of my two oldest boys, but my ex still wanna see my daughter and spend time with her, because in all this years, they build a father and daughter bound. He loves her, like his own child.