How lefthanded is your friend? Does he throw left, bat left, eat left, hammer left, pick up items left?

Most likely he is a mixture. People who are 100% left-handed are rare. I believe only 5% of so-called lefties are 100%.

When your friend writes, does he curl his left hand inward? I have read that people who do that have a right-hander's brain. I'm one of them, and as far as I'm concerned it's probably correct.

If I were your friend, I would start playing rightie, and only switch if it just feels awful.

I am supposedly left-handed, but I use my left only for delicate tasks such as writing, embroidering, and washing contact lenses. I play guitar rightie. Guitars are big.