Hi, I am visiting India with a friend who uses crutches. What are the things I should take care of?

First thing is try to get an upgrade on air seats.

Little things can make a big difference. If you haven't decided on the hotels, call up your shortlisted ones and ask about elevators, ground floor rooms and the general path textures. Some resorts go for the rustic feel and make cobbled pathways, which might be a bit of a nuisance. Checking hotel grounds photos beforehand should give you a general idea of accessibility too.

The sightseeing of monuments could also be a bit problematic. Most of the historic forts & palaces do not have elevators. The best way out could be to limit your choice to the must-see. Getting on to the jeeps for wildlife safari could be circumvented by asking your agent to provide a sort of a sports-victory stand to climb into.
However, this disability must not deter you! You will go back very happy with a little homework before arriving!!
Happy travels!

You don't mention where in India and when you will be there. Walking rural areas in the rain might be a little bit of a problem with the bottom end of the crutch stuck in mud or worse stuff at times.

On the positive side, if departing from most Western airports - passengers with mobility issues get to get on the plane first.

Good luck.