Married to an Indian immigrant 30 years. Have learned a lot over the decades. So many things I can touch on. I only scanned other posts.
First- is she from India or was she raised here is an important question. Then- which state in India is she/are parents from? This matters because practicing Hindus from different areas follow different dietary rules (H atheist but avoids red meats as a cardiologist). Also typical foods vary in different parts of India- just as they do among Europeans and different US states.
My H’s adult niece is vegetarian although she has done chicken in the past. She stayed with us a few extra days recently when she came with her parents. Found out she doesn’t like to eat Indian foods- her mom is a good cook but does other ethnic foods along with Indian. I like to cook and can do good Indian food. But- didn’t for her since she did NOT want any. Chinese (tofu protein), Mexican, Italian, Greek (falafel), peanut butter sandwiches, hummus, eggs in veggie omelets (some do/don’t eat eggs) worked for restaurant, carryout and home foods while she and parents visited. She likes cheese as well.
The young woman visiting you expects to be introduced to YOUR family’s culture. Serve the foods you normally would with alternatives if you are serving the family’s favorite beef or other meat based dishes. Find out if she eats chicken and fish. I make fajitas with black beans and chicken separate from the veggies- that way vegetarians can avoid the meat. DO NOT avoid meat for the rest of you.
We went to a Jain-Hindu wedding several years ago where the parents (friends of ours) served mainly vegetarian entrees but added a meat lasagna from their caterer for the meat eaters. Indians in the US tend to be low key- they seem very accepting of others’ differing habits.
Remember- she can get plenty of Indian food at home most likely and would enjoy other foods.
Gifts- the amount to spend I thought I read sounded like a lot to me. But that depends on your budget and spending habits. Think of the same sort of gifts you would for any young adult woman from the US. My niece chooses a red shade of lipstick and she is light skinned- like some Italians and Hispanics and others with Aryan roots. South Indians usually have darker skin than north Indians. My H can tell by last names which region Indians are from like we usually can detect European origins from names.
Just be yourself. She and your son like each other well enough to visit you so you already know she is comfortable with him. US born/raised means American. Indian studying in the US- accustomed to college life here plus learned English growing up. Current day city life in India a lot more parallel to US tan decades ago when H came- they have caught up dramatically.
My mother-in-law likes several American/non Indian ethnic style foods- cheese pizza, pasta, French fries, bean dishes, falafel, etc. She spends a lot of time in the US (citizen so she can leave here for longer than 6 months) but still considers potatoes a vegetable. She was used to a much greater variety of vegetables in India. Several days in your household will not cause problems for your guest’s nutrition if she finds your foods do not taste good. I tend to eat meat for lunch at a nearby fast food restaurant when we stay with my sister-in-law since she seems to serve carb heavy foods (eg beans) and I need more protein-less carbs. Your son and his girlfriend will find meals away from your house during her stay as well.
Relax and enjoy.