When he was d2 he had a d5 duo and as a d1 he has a d4 duo. As per ranked duo restrictions that is allowed.
"Diamond V: Can only queue with Platinum I, II, III, and Diamond II, III, IV, V Tier players.
Diamond IV: Can only queue with Platinum I, II, and Diamond I, II, III, IV, V players.
Diamond III: Can only queue with Platinum I, Diamond I, II, III, IV, V, and Master Tier players.
Diamond II: Can only queue with Diamond I, II, III, IV, V, Master and Challenger Tier players.
Diamond I: Can only queue with Diamond I, II, III, IV Master and Challenger Tier players.
Master Tier: Can only queue with Master, Challenger, and Diamond I, II, and III Tier players.
Challenger Tier: Can only queue with Master, Challenger, and Diamond I and II Tier players"
Chances are because of his habit of duoing with someone at the absolute lowest division the game allows, his mmr is probably pretty messed up. I've seen it often where someone has X rank but play with X rank minus 3 or 4 divisions and the common denominator seems to be that they duo a lot with people of the lowest rank allowed to duo with.
It is kind of a way to cheese your way to a rank. A see a bunch of plats duo with gold 5 so they have to beat high gold / low plat to get into diamond instead of earning it vs their rank and the rank they are going into.