The single friends will envy your girlfriend and try and take her man away. If they cannot do that, they will look for negatives in the new boyfriend until they cause problems.
Generally, since I'm only looking for short-term girlfriends, I pay close attention to the friends. With this girl I'm about to break up with, one of her friends is very negative about single men, the other is desperately trying to find things about me that she doesn't like. Eventually, I will have to make a stand and confront this issue with her friends. She won't confront them, so I'll have to break off the relationship.
She gets her friends back; and they are all single again, so they are all Best Friends Forever yet again while they wait for some perfect guy to come along. Of course, they'll go back to screwing every guy they meet for a while, but I don't care. I'm long gone by then.
Women's friends will do anything to help destroy their friends relationships. It creates possibilities for them, while keeping all the girls single; so they will be able to share the same experiences.