Hi, I don't have direct parental experience, but I have a young female relative who displays many Aspergic behaviours and always has (no ASD diagnosis, but she is sooo ASD!) She went through a plump phase, then as a teenager, went into a terrible decline and became anorexic. Not sure why exactly, but a combination of not coping socially at school, comments about her weight and just not getting proper recognition of her needs and ASD issues). She is doing better now, but years on is very underweight, is unable to hold down work and has major food issues (as well as an ever-changing mental illness diagnosis). Anorexia gets a dreadful grip.
So, like you say, the weight thing should be handled very sensitively, but of course it needs to be tackled.
Do you know if the eating disorders charities like BEAT have any information on handling food issues sensitively? Otherwise, being very careful what sort of food you have in the house (e.g., no family packs of crisps etc.) and trying to get her to eat - low fat - protein (keeps you fuller longer), and high GI/GL foods that will keep her blood sugar levels more even. Work exercise into the day without her realising, e.g., some motivation to walk somewhere for 20 minutes daily. Can she walk to school? That is often the way young teenagers start shifting the fat
Hand out a large bowl of crudite and low fat dip and a glass of water before each meal. Could she help cut it up and prepare it to get her used to enjoying healthy food?
Hope I'm not teaching Grandma to suck eggs!!