People like this are annoying. That is, those who can't function independently of their partners.

I had a friend like this. We were best friends in high school. Fast forward to a few years ago. I realized I hadn't hung out with her one on one in over TEN YEARS!!! Don't get me wrong. Her partner is great. However I don't think it's too much to want to see a friend one on one every so often.

You may have to accept that this is just how it is.

But word of caution. If you can only have your friend as a package deal, don't let her come running back if she suddenly becomes single. That's not fair to you. She'd essentially be using you as someone she hangs out with only when she has nobody better to be with.