if you play on his account in normals when he is offline or whatever to level up his accounts, technically yes its against terms of service.
it wouldn't matter much since his account getting banned probably doesn't have much on it, but none the less that is against the rules.
the leveling system is designed in a way to get players some experience before they go into ranked or whatever. the problem is the leveling system is filled with tons of smurfs and until your MMR rapidly lowers to where it should be with other actual new players your first few levels you will just get continually assblasted.
Its up to you, but account sharing can and will get your account suspended. I know it got one of mine suspended. so i dont recommend it to anyone. just tell him to be patient.