Denis and I are both vegan. We always get a kitchen, preferring not to be completely dependent on restaurants. We only eat out occasionally.
As mentioned above, your friends should be familiar with Happy Cow. If they're not, they could use their home town for practice.
A few years ago I posted a review of La Tecia Vegana: https://community.ricksteves.com/travel-forum/italy-reviews/vegan-in-venice
When I looked for it by putting Vegan in Venice into the search engine here, I got 36 listings: https://search.ricksteves.com/?utf8=%E2%9C%93&query=vegan+in+venice&button= Of course, they're not all current or solely about Venice, but they could look at those threads for some reassurance. And they won't all be the token vegetarian salad special, lol! Or just pasta alla pomodoro.
When I studied Happy Cow listings, looking at menus, I noticed that the "contorni" section of the menus looked like one could do just fine, and be comfortably full.
One of the things that bothers us is the scarcity of suitable dessert options. Never fear, sorbetto the the rescue! There are plenty of vegan options there, yay!