You don't. She is definitely not your friend. BUT, the big thing is that the guy has taken up with her, and that is his fault. So, sadly, that is the situation. You are better to seek the high ground and move on from the two of them.
ANSWER:
short version of what we can call those women that dated a married men instead of single man is " mistress" or " home wrecker."
The classical answer is 'homewrecker'
Il cornuto?
A jackass!
valet
Elektra?
No, it is not alright to call another married man baby. The word 'baby' is a nickname you should reserve for your husband.
I believe you would call them a Suitor.
Lita Ford - Your Wake Up Call
A liar.
what do you call woman who want to vote
A woman who tries to steal another woman's man is often referred to as a "homewrecker." This term implies that she is attempting to disrupt or break up an existing relationship. Other colloquial terms might include "mistress" or "interloper," depending on the context of the situation.
She is just another woman.