Being away at college tends to spread out friendships and it can be really difficult during a time of need. While it's usually impossible to drop everything and be there for one another, there are still things you can do.
1. Send a letter.
2. Send memories or pictures of memories.
3. A phone call - even if it's 5 minutes.
4. Surprise them with a dinner delivery.
5. Plan for the summer (or the next break you'll both be in the same area).
6. Or plan a trip together.
7. FaceTime movie date or dinner date.
Just because you can't physically be there for someone does not mean you can't be there. So many little things play into making someone feel better. Take what you know about the person and utilize those things from a distance. The little things one wouldn't think of can make the world of difference.