What Can I Do To Help His Kids?

I live with my partner of 2 years and my 5 year old son. My partner has 2 kids with another woman and i love them very much. they come over every weekend and stay overnight every other weekend. However i have a few issues with them.
His son is 6 nearly 7 and has nothing right with him. By this i mean his mother insists he had ADHD so he cant learn properly at school, she says dyslexia comes with this so he learns much slower. she has been saying this for quite a while now but he is still not bein medicated for it and with family members of mine with ADHD they have been medicated very quickly after diagnosis so it makes me think if she is lying. i know how bad that is and i shouldnt think it but she kicks up a fuss when my partenr asks to see the letters from the doctor. now she is saying he has an eating disorder which also come with this problem. this means she can only feed him junk food (pizza, chips, frozen potato products and reformed foods) as that is all he will eat. He has always been a picky eater refusing to eat veg or potatoes which werent fried or covered in fat and try wont new foods. now he has stopped eating bread saying he doesnt like it. She says she has tried but he just wont eat it although he has done for the last 6 years. she also says the doctors told her not to push to get him to eat healthy food although no letters have ever been shown for this or anything else. when we try to get him to eat he sits and crys untill we cant take it and take the food the away.
she printed some info off for my partner about his condition but it was about two different conditions one saying minimal brain damage.
It seems to me she opts for the easy option each time and gives in to him. giving him other things he likes more? I also thought it might be an attention thing, she is a single parent and has been since the relationship between her and my partner broke down. she has 3 now and expecting another but still no help at home. it must be hard for her but it annoys me that he is getting worse and not better.
He never wants to get involved when we do something as a family like play a game or make cakes or our own pizzas he would prefer to sit on the computer all day. he craves and seeks attention more than most. he is behind with school work, seems blank when you talk to him somtimes like he just isnt there and when you tell him something he forgts straight away.
Could anyone tell me what is an actual problem that can be treated and what isn’t so we can start to help him.