Friendship isn't always 50/50
Remember that while your friend is going through a challenge, staying in touch with you may be difficult, for many reasons. Don't expect responses - but do continue to keep the communication going! Frequent emails and cards through the mail will let your friend know you care.
No Replies Needed!
When leaving a voice message or a written message, be sure to let your friend know you don't need a response, you just want your friend to know he/she is on your mind today.