MMR is MatchMaking Rating. It's basically a hidden ranking that affects how much LP you gain/lose upon loss, and where you will be placed. I believe MMR is separate in norms from ranked, so your friend is wasting his time.
For example, I play a lot of norms with my IRL friends, who suck (~silver rank), so if they're too heavy and we lose, my mmr tanks. I still get matched ~gold when i play by myself because of that (I'm high plat). In ranked, my friend was plat 3 after preseason, had plat 3 mmr, and went 5-5 in placements and got Silver 1. I was plat 2, had Diamond 5 mmr, went 4-6 (1 division up, 1 more loss on placements) and I got placed in gold 1. People in silver doing provisionals are most likely going to get placed in bronze regardless of record in games because of this, and there isn't much you can do about it now. Your chance to boost mmr was in preseason, you're going to have to grind it out one way or another.