She has reached the age of puberty but she is not strong enough to fast

If the situation is as described, then she has to fast because menstruation is one of the signs of puberty for women, if it comes when she is nine years old or more. If she is able to fast then she must do it at the proper time, and if she is unable to do it or she faces extreme hardship in doing so, then she may not fast but she must make up the days that she did not fast when she is able to do so. End quote.