Doesn't matter whether it's a girl or bloke, frankly it's a large amount of time to spend with another person when you have a family and a relationship.
What is he getting from this relationship that he cant' get from you, and are you giving up a little bit?
I would organise your time a little bit so you know when she is coming round, or they are going out. EG when are you seeing such and such, this week, and stick to the times - add them to a calendar if need be. Then if she just pops round unannounced and parks herself in the living room you can say, DH and I made plans to do some things this evening, he's seeing you tomorrow, so we'll see you then???
My Dh had a lot of lesbian mates as he was in the army, generally I loved the fact that he had great friendships, and hoped he'd understand the female perspective a lot more, in such a male environment. However there was one girl who used to push me quite a bit, and disrespect me infront of the group of friends. She thought it was hysterical to ask my husband to impregnate her as she wanted a baby, and saw how cute my son was so wanted the same. Thing was, it went on and on every time and well, really wasn't funny. I told him she couldn't come round anymore as she was a prick, and he kept the friendship to work only.
On the whole it sounds like you need to do more together, and have more fun together then this woman will fade into the background a bit.