She's ten stone plus overweight, this isn't a case of someone liking their food, you don't get to twice a healthy bodyweight without having extremely disordered eating and an inactive lifestyle. She has eaten herself into infertility and a perforated bowel. At 32. No child should have that diet inflicted on them!
She can't take care of her own health, how on earth could she possibly provide healthy meals and appropriate activity for a child? Not to mention that her size reduces her mobility. How would she deal with a child who runs off? It's not like she could run after them. She couldn't grab a young child running out onto the road. Her reflexes wouldn't be fast enough.
Big, healthy people can foster, nobody's rejected on bmi alone, it's the health and lifestyle. She can't foster because she's barely mobile and suffers health problems due to her eating disorder and lifestyle.
Foster care is about providing children with a safe environment. You need to be able to feed them, exercise them, keep them physically safe, nurture them, deal with behavioural and psychological issues they may have. It takes a very competent, mature adult who can see from the childs point of view to do that.
Not someone like this woman who can't even take care of herself or understand that it's not all about her and her sense of entitlement.