Tenant smoking weed is n his room

We are currently rent a house and have a tenant renting a room that is smoking weed up in his room. Being a previous smoker myself (still have one on the odd occasion) the smell never really bothered me too much and the same with the other housemate. We had a spare room in the house so we decided to advertise the room and a lovely Japanese woman (about 40) has decided to take the room. So I had a conversation with the tenant that is smoking that the smell is no longer acceptable and he needs to make an effort to keep the smell out of the living area which he agreed with. I’m aware the Japanese are respectful people and I want to be the same towards her.
Over the last month I have had numerous conversations about the smell in the house as he is smoking with the windows closed and have asked him repeatedly to open windows and at least try to blow the smoke out but he continuously fails to do this. So today is the day that she moves in which he knew and I walk up to the middle floor of the house and it stinks of weed so I sent him a message giving out and that I’m not going to keep having the same conversation, told him to seal up the inside of the door some how and the if it keeps happening, things won’t end well.

So I’m looking to see what’s the best way to deal with this if it keeps happening as he is my friends girlfriends brother and I don’t want to make things bad but at the same time I don’t want the new tenant to be uncomfortable in her new house.

TIA