"her dad is not her actual biological father"

Who cares. he is her dad.

But seriously, while pedantic I think differentiating between "dad" and "father" is part of the solution here. Your title leans into one of the most abrasive ways she could break the news (I know that wasn't a proposed phrasing). "Your my dad, I love you, but turns out your not my bilogical father" is much more palatable way to break the news, and puts the focus on what's actually important.

Either way, as you said, it's her decision. Even if agreement from us commenters is unanimous she should chuck out all our opinions and do what she thinks is right.

When Digg put business above users and reddit growth exploded it (reddit) wasn't a great "alternative" at that time either. But at that time they were putting users first. Isn't that what makes this (lemmy) a great alternative?