She tested it by attacking him herself, and spells seem to do almost no damage because of his regen. But then she attacked him with her bow and she practically killed him in a single hit.
Now, she has no points in any archery related skills, and is lower level than I am (58ish compared to 63 when this test was done), and her bow does less damage. I, on the other hand, have completely buffed my archery in every way possible and when I tested it on my game, am still barely damaging him.
It's basically come to the point where she can't take him into battle at all because he dies so quickly. We read somewhere that Shadowmere respawns after like 15-20 days if he dies, so she tried that, but unfortunately after waiting like 25 days, he still had not shown up where he died.
Any one have any ideas? She's starting to get a bit frustrated with it because she liked bringing him into battle with her.

