I learned this lesson a long time ago: Never, ever, give your companion a fatman, missile launcher, or a grenade if you want to live for long.
Only companion ATM that I trust enough to carry a fatman is Dogmeat. Since he doesn't have arms and hands, it's kind hard for him to fire it when I least expect him to. Even so, when I do give them one, I refuse to give them ammo for it too.
Side note: Once, while playing FO3, I mistakenly gave Charon a Stealth Boy. A few minutes later I turned around to say something to him, only to find he had disappeared. WTF, Charon, get tired of me already? Nope, he had used the Stealth Boy immediately. No enemies nearby, no need to use it, he just used it for the [censored] of it.