Putting limits on his relationship with her is not the way to go here. Instead, focus on what you need. If it is a dedicated date night for the two of you, then do that. But telling him he can't see her isn't fair to either of them. She may need 2 or 3 dates a week to feel connected, while maybe you would be ok with only 1. Don't compare what she is getting with what you are getting - instead focus on your needs and go from there.
A modern day polyamorous polygamist My Online Journal
Me, Central Hub in a Y shape (38F)
Wife to DarkKnight (40M) & PunkRockAwesomesauce (42M)
Girlfriend to WarMan (40M) - Monkey is his long time friend