I can understand the arguments about the difficulty of caring for a large sized infant (essentially). However, the treatment was such a drastic step to take... I just don't feel comfortable about that.
I do support the parents in getting the girl a hysterectomy. This child may be disabled enough that she may only have brief contact with outsiders, but there are people who prey on the disabled. In addition, they have quite enough to deal with without having to add menstruation to the list.
I had a female cousin who was severly retarded and crippled, basically an invalid. She started her period around age 13 and lived to age 25. Her mother and grandmother had to deal with that along with feeding and changing her. They went through a LOT. Thankfully, she was put into a nursing home around age 18. The mother worked at the nursing home and the grandmother visited several times a day. THe situation was SO very sad.