Just let your friend heal himself, I don't find it necessary to heal during combat in this game and it's easy to set up a poison field to heal up in after combat. If the zombie perk is really getting in the way just tell your friend that and reload an earlier save or start a new playthrough. I think zombi would work best when you are using rain/rain of blood + contamination all over the place and have all party members with the perk, not all perks are are powerful as each other and some need you to build around them. In the same way a stealth focused build works a lot worse if you have party members that aren't using stealth, zombie doesn't work well when not everyone has it.
