My parents aren't very hardcore Hindus, but many of our family friends and relatives (including my in-laws) are pretty strict about the Hindu brahmin diet - tons of dairy, rice, vegetables, legumes, and flour-based stuff (chapati, papad, etc.), avoid eggs (unless they're an indistinguishable ingredient, as tcdup mentioned), and eschew all meat & meat-based products (no animal-derived gelatin, no lard, some know about & avoid animal-derived rennet in cheeses, etc.).
If they're just plain ol' Hindu vegetarian, they'll eat onions, garlic, etc. If they're adherents of certain sects or followers of certain gurus/swamis, they won't eat those, but will eat other root vegetables. If they're Jain, they won't eat anything that, when harvested, would have resulted in the death of the plant (a.k.a. no root vegetables or onions/garlic).
When my in-laws visit, we don't really eat Indian food unless MIL cooks. They're perfectly happy with pizza, Mexican, Thai, and veggie subs/sandwiches. However, after a few days of that, they do start to crave Indian (specifically, her homemade Indian), and we're happy to cede the kitchen over to her and enjoy the results.