Long read but I do need advice!

This is for a friend. I am not in this situation but I would like some advice because I feel truly bad for him!

So my best guy friend has been in a relationship with a girl for 4-5 years now. Just to make it easier I'm going to make up names for them. I'll call him Bob and I'll call her Kate (not their real names just keeping it anonymous!) They have been engaged for 2 years and have a 1 year old together. They have an apartment together, both have jobs, pets, very independent and take care of their family.

Before Kate got pregnant she would occasionally smoke weed. I wouldn't say she was an addict. She probably smoked twice a month just for whatever. When she got pregnant she cut cold turkey for the sake of the baby. Fine.

Their child is now 1. Kate started casually smoking again every once in a while. But for the last 4 or 5 months Kate has been smoking a lot! From maybe once a week to 5 times a day. She says that she needs it for her anxiety but she refuses to go to the doctors to get checked out. She will smoke while watching their child. She puts weed above her child.

For example. When the baby was 6 months, Kate left her in the car, went into her mom's house and smoked for an hour. Another instance is Bob caught her smoking in the house with the baby laying beside her. She does it while driving with the baby in the backseat. Bob told her she needs to stop because it's affecting her health. She's weak. Has these flashes and passes out all the time, goes ghostly white and her whole body shakes when she doesn't have access to weed or she's angry or something.

Yesterday Kate and Bob were arguing he said he needed her to stop with the weed especially around the baby. She said no and he said then we have to postpone our wedding she was really mad punched a wall in their bedroom and went out to the living room area. Their child can walk and say 'mama' and the child told Bob they wanted her. He said Oh she's out in the living room go see her. He didn't know that Kate had left the apartment. So he assumed that the child was with her. He went out to grab a drink and the baby was sitting outside on the balcony to the apartment building 5 stories up. He was so upset. Because she didn't agree with him wanting her to stop smoking she left without telling him and he found his child on the balcony railing. (The baby is ok!) But still he's so mad.

What kind of advice would you give Bob in this scenario. Help!