An answer from Mary:
It’s sometimes hard to support a caregiver, given the heightened loneliness and constant demands of caregiving (and the difficult emotions that arise), especially when the friendship is strained to begin with. It sounds as though your heart is in the right place, wanting to offer what you can, but you’re also feeling resentful of things that have little to do with your friend’s role as a caregiver—more how she has treated you as a friend.
The bottom line is that if you mainly feel resentful, and you’re having to make constant adjustments to protect your own feelings, you’ll burn out as the caregiver to the caregiver.
Your friend needs all of the support she can get, but you do have to step away to recharge your own batteries. Let yourself off the guilt hook for needing this time.