I'll add that I'm not the smallest guy around (5'11", ~310lbs), and while I have no issue fitting into a 17" RJ seat, the issue I have in the A seat on the ERJ145 is that the slope of the fuselage seriously cuts into my head shoulder room, forcing me to lean into the aisle on an almost constant basis. I realize that OP's friend isn't quite as tall, but the slope might still make the A seat uncomfortable if the friend is broad-shouldered. I'll echo the others and say to take the aisle on the BC side and hope for the best.