Milk is not main ingredient in South Indian dishes. Most might be vegan.

They use a fair bit of ghee don't they? I went from memory but Wikipedia seems to agree with that.

"Ghee is widely used in South Indian cuisine for tempering curries, in preparation of rice dishes and sweets. South Indians have a habit of adding ghee to their rice before eating it with pickles and curries. South Indians are one of the biggest consumers of ghee."

While everything tastes better with ghee, you can cook a lot of Indian food using plant-based oils. I use sunflower oil for a lot of dishes, but you can also use vegetable oil, olive oil, or mustard oil.

With a bit of effort, you can eliminate animal products from most vegetarian Indian food. It will taste different, but not too much.

Source: I'm Indian.

I don't think anyways arguing that oil isn't a replacement for butter. I think we're curious about the expectation. Can we expect most Indian restaurants or caterers to use oils instead of ghee? Do you need to ask them if they use ghee?

Fancy Indian restaurants in the West as well as back here are almost certainly using ghee. Cheaper places might use vanaspati (a ghee substitute made with palm oil) or oil. In general, though, you can never tell for sure unless you're doing the cooking yourself.

Coastal Karnataka, Kerala Coconut oil is used for tempering. Really any oil can be used for those curries.