Vegetarianism has been a (mandatory!) prestige marker in India for at least several hundred years; a traditional diet of that type will be adequately healthy[1]. What's hard is cutting meat out of a meat-based diet. You can't expect to just intuit how to fill the holes it leaves.
[1] As with all traditional practices, partial adoption may result in loss of any amount of the benefit, including amounts greater than 100%.
Also applying diets from one genetic cluster in a different one from another climate is asking for trouble. I can process lactose fine, but many can't. Chickpeas and lentils I do not absorb well.
It is a risk, but it's still true that the Indian vegetarian diet will cover all major dietary needs. And while that's not the most ringing endorsement, it's more than you can say for cutting meat out of a diet that features a lot of meat.
How much dairy protein is in such a diet? A lot of the curries I enjoy have yogurt and I love the cheese and cream in palak and saag paneer. But those may not be normal dishes?
I don't know, though my first guess would have been that the major source of protein would be lentils or other pulses. Yogurt is certainly plausible as well.
In England eating beef has been a prestige marker for at least several hundred years[1]. A traditional diet of that type will be more than adequately healthy. What’s hard is getting over ancient superstitions about animals being reincarnated people and then rationalizing that it’s actually healthy for an omnivorous species to abandon eating meat.
Or maybe there’s no reason to believe that a diet that’s healthy for Indians is necessarily healthy for Europeans or vice versa.
[1] As with all traditional practices, partial adoption may result in loss of any amount of the benefit, including amounts greater than 100%.