Wa'alailumussalam warahmatullahi wabarakatuh, dear brother.
I'm in love with a woman and she also loves me a lot. But she is married. Her husband gives her rights.
I would like to remind you to be careful brother since having intimate relationship with someone else's wife is haram and prohibited in Islam. Be careful with satan trickery.
He is always making her sad and he is having affairs with other women
which is zina.
You can help her by making sure it is a valid accusation. It is a serious accusation towards any muslim. Make sure that you are not easily come into conclusion without any proof.
how can she get divorced?
Divorcing married couple is a sin. All you can do as a third person is helping her case reported to reputable people for help. She can then ask a judge opinion to divorce her marriage.
You can ask her for marriage once she is no longer someone's wife (and finished her i'ddah).