@ the OP - Healing Hands is the spell you want. Most merchants who sell spell tomes should have a tome you can learn it from available.
I had Lydia killed a few days ago fighing a NASTY Lich style undead. They can die.
In the case of proper followers, only if they're hit by friendly fire while they're down to their last sliver of health and kneeling on the ground. When they go into the kneeling position, all enemy aggro immediately ignores them and switches to you instead, and outside of a freak glitch (I suppose) they cannot be killed by anything except friendly fire. You or another one of your followers hit her without realizing it, most likely. (Possibly by Shouting--those Shouts have a very wide blast radius; I've accidentally killed Erik once or twice this way. Hooray for the reload button.)
It's supposedly possible for followers in the "recovery" phase to be killed by an enemy AoE spell or Shout or the like, but I've never seen this happen in practice--and I've been in fights practically on top of a kneeling/recovering follower. Only times I've seen them die, it's been 100% my fault.
If he is a follower, just let him go down. He'll get back up, they always do. Unless they get hit while they are down.
Maurice Jondrelle is a quest-given follower and isn't essential, so that particular suggestion won't work too well I'm afraid.