Let him know that using a toy is an absolutely different experience than having sex with a partner, even if it's a super realistic penetrative toy. It feels different, it's psychologically different, everything. It's not about being replaced, it's about expanding the repertoire. One thing I say to similarly-threatened men who come into my sex shop is that try as they might, men just cannot vibrate. And vibrations feel really cool. And women don't hold it against men that they don't have this super power, they just augment their experience with bullets and rabbits and the like.
For a first toy, especially if it's an internal vibrator, maybe have your friend look for a small, unintimidating toy that won't be threatening in size to her husband. Way back in the day when I got my first vibrator my boyfriend of the time felt a little weird and threatened, until he saw it. Once he realized that his penis was bigger than my toy (and that me wanting a toy didn't just mean I was yearning for a bigger wang) he quickly relaxed about the whole thing.
Toys can also be used together to make it a coupley experience, which might take the edge off. The We-vibe might be a perfect place to start