She's never going to believe you. Period.
When they have it in their head that these friends aren't just friends, there is nothing you can do or say to make them change their minds.
Honestly, if this is an issue, you need to 'kick her to the curb'. Trust is what defines a relationship.
It's obvious that she's a child if she's complaining about 'female friends'.
Lol, then they had bad relationships. It's as simple as that.