I have a friend who has been married for about 10 years now, and they not only sleep in different beds, they maintain separate bedrooms. They just both like to have their own space. I don't know that they don't ever sleep in the same bed, but it is what works for them.
I think I'd be the same way. I have just never really had any interest in sharing a bed with someone. It is a specific and particular type of intimacy. Some people want it and others don't. As long as it works for both people it should be fine.
It could also, as others have noted, simply be an issue of comfort. Some people have a hard time sleeping. I am one of them. For me to sleep, I have to have very specific levels of noise, temperature, light, etc. Even small variations can keep me up all night. Perhaps they just couldn't find a happy medium.
It probably wouldn't hurt to ask her about it. Maybe it is an issue for her and a friendly ear would be a nice thing.