There are so many possibilities, it is hard to say what is doing what in your friend. Risperdal is usually associated with a lack of a period, not a cause of one. Your brain may think you are nursing because of the effects on a chemical called prolactin, and you can actually lactate. This is not all that common, and the finding of causing one to menstruate would be uncommon, and I have not heard of it. The weight gain is probably caused by the medicines and inadequate treatment of the illness.
People eat sugar to push more tryptophan in their brain to make serotonin.
She probably needs more Paxil. I doubt the Risperdal caused all the weight gain by itself.