If my friend married an American man who gave her a green card but he is guilty of adultery and they divorced, is she still eligible to file for a her next green on her own?

If the condition on the green card has already been removed, your friend can renew her green card without any help from her former spouse.  If she is needing to remove her conditional status because the two-year anniversary of their wedding is coming up, then your friend will need to apply for a waiver based on divorce.  She will need to prove that the marriage was bona fide when it was entered into.