How to explain a friend that his relationship is killing him/her?

We all have a friend that was normal until getting in the relationship with a girl of his choice. I see it all the time, a lot of friends just cantt handle being separated with their better half. Its not that their boyfriend or girlfriend is not a good company, just that you know a friend for years and you cant even get it on alone drink to talk and see whats new. A lot of them just dissapear or show up with their love every single time.

Have you ever had that problem?

A friend of mine just called. A few of us were arranging a drink, school buddies only. She automaticly called her boyfriend - and thats fine with us. Problem is that she only wants to go when her boyfriend can and we have a different working hours with him and can not schedule the drink.

Should we tell her or wait and try to get together when they both can? What is your opinion?

Have you ever been in that kind of relationship and realized that later, after breaking up? I never had that problem and I think that is the reason i cant understand it. Is it in human character and cant be changed or is there a way out?