This had me double-checking the rulebook, so I'll just quote it here:
"Discard to heal." Spot a unique companion or unique ally with at least one wound and discard a card from your hand with the same card title (it may have a different subtitle) to heal that character.
- You can discard to heal only unique companions and allies.
- It looks like you can discard allies to heal companions and vice versa, since the rule only checks for the same card title. However I'm not so sure about followers and other card types based on the FotR rulebook from way back:
Discard a unique companion or ally card from your hand to heal a character who has the same title (it may have a different subtitle).
The question is, was the rule intentionally updated to cover other card types? Since the Comprehensive Rulebook supersedes the FotR rulebook, it looks like discarding followers are okay. I'll see if I can dig up more stuff on this, though.