I'm no expert, but I spar with my class mates after our workout. This may sound funny, but I try to imitate Mayweather, but south paw. I hang my right at the stomach and use my left to cover my face. My body is turned to the left away from him. For the first minute I'll just lay back and let him throw. I'll throw jab to keep a distance, but nothing major. I don't know if your friend is new, but he reminded of a new guy I spar with one time. I was orthodox this time cuz I didn't think I could go south paw with him. I only go south paw when I spar with guys at the same size (naturally right handed btw) He is taller and have more reach and he love throwing jab to keep a distance. What I did was just put my hand up and eat the jabs. He will eventually throw a right cuz he feel confident since you're not doing anything. You need to time this and memorize the pattern. The guy I was sparring with throw the right when I take 2-3 jabs and within range. So what I did was push in and pressure him taking all the jabs. Hes not fast so I can see him setting up the right with his body movements. He also like to leave his left hand out while throwing the right. So when I see this I'm waiting. Just when he throw it, I duck under into him and throw a over hand right. It connect very nicely. I keep doing this until I hit hard 3 times and he pretty much stop jabbing and no right at all. Guys who does 1 thing a lot breaks when you counter it a couple of time. They'll get scared to do it again and cuz you will smash him.
Neway, when I'm sparring guys my size and doing south paw. I like to keep my stance low and learn my head back leaving my right shoulder up high. I find it easy for me to back off out of reach when they're throwing punches. I find that some of them gets frustrated and commit to their punches and fight as though I'm the shorter opponent. Surprise them every now and then with a straight from a higher angle.