Every time our friend gets a boyfriend she disappears from the group and then when they break up she's back. What's the best way to tell her we don't want to play second choice to whomever she's dating?

It's interesting how some of our friends seem to lose their entire identity as soon as a romantic relationship enters the picture. And they immerse themselves in his life and forget about or at least set aside those things that would've been important to them in their single life.