I would come up with cute, fun things you could send her every other week or so, such as a card, a book, a movie, flowers, etc. Appoint yourself her fairy godmother and send her all the sweet, wonderful things you'd like to get if you were going through a tough time.
If it were me, I'd start with a "I'm glad we're friends" card. Then I'd send her a copy of my favorite book. A Starbucks card for a fun drink. Send her a snapshot of you two doing something silly from way back when or find a meaningful poem. The gift of words--funny, inspirational, heartfelt, whatever--will be something she could go back to over and over. I did this for my best friend after she had her second child and was going through a rough patch. My friend loved just knowing I was in the background caring about her.
Good luck.