My [20F] boyfriend [30M] has a severely constipated 4yo and won’t listen to the doctor

My boyfriend and I have been seeing each other for about two years, and dating for over a year, and he has a four year old daughter with a mom that’s not really involved. The daughter has been having giant poops that she struggles to push out and struggled to be potty trained with pooping on the toilet which I’ve been telling him she’s constipated but he wouldn’t listen and would get offended. He lives with his parents and they feed her tons of fast food and cheesey foods, candy and sweets like little Debbie’s type things, give her a cup of milk at the start of everyday, and juice everyday. She constantly would just not eat the food she’s given and then immediately ask for snacks like chips or cereal and candy and sweets and my boyfriend would give it to her saying “I’m not going to let my kid starve”. They recently went to the doctor and he told them she’s so severely constipated it’ll take six months of following a diet for her colon to return to the size it should be for someone her age. He said less dairy and cheese, less fast food, obviously just generally unhealthy stuff, and more fruits and veggies especially fibrous things like raisins. Within two days his mom started feeding her cheese and sweets again and she would just smile and shake her shoulders because she thinks she’s being cute when she does that and it excuses her actions. They’ve just gone back to feeding her fast food everyday, constantly giving her chips and candy, cheese and cheesey foods, and the doctor said less milk but they always start her day off with a glass of plain milk mixed with chocolate milk and they get mad when she doesn’t finish it so she ends up chugging it every time. She’s obviously still constipated and I have no idea what to do I love her and hate seeing this happen but I’m not the mom so it isn’t my place to comment on their parenting but it’s so hard to watch this happen so I just was wanting advice on how to talk to my boyfriend about this not being the mom or directly part of the family but I’ve been around and helping out with his daughter for the past two years although I typically only come on the weekends.

TLDR; My bf has a severely constipated 4yo and is ignoring the doctor’s advice and idk how to talk to him about it not being the mom.