I'd like to get my companion to heal the player. How do I go about setting this up and making sure she casts it on the player when he's so many points below normal? I also want it to be higher priority than fighting.

Not sure how to go about this in Skyrim. Are there any how to videos out there? Has anyone done this?

Any help vastly appreciated.

I think you'll have to setup an AI package within a quest alias with a Usemagic template and Healing Hands selected as the spell object type. I believe the NPC Faralda does something similar when the player first arrives at the college of winterhold. Faralda leads the player casting magelight spells along the way. Don't forget to set some important flags, like Ignore Combat and possibly No Combat Alert.

I have done something like this before, but not with Skyrim.