That said, the GIRM clearly uses 'viri', that is 'men' for the Washing of the Feet. This is just an option for Holy Thursday and need not be done at all. However, if one is going to do it, one should stick to the General Instructions. If the leader does not, it simply gives license to all to do exactly as they please.
Further, washing the feet of a 12 year old Muslim girl might not be the wisest thing to do.
It's likely you know more about canons and rubrics than I do (it wouldn't be hard to know more than I do, because this is an area of vast ignorance on my part!) But I was under the impression that a Pope's liturgical directions would take precedence over anybody else's, except for things that are strictly dogmata, e.g. the validity of the form or matter of a Sacrament.
Is not the Pope the chief liturgical authority of the Church? Does he not have this authority?
Why pray tell? She does not need Jesus to wash her clean? That one act may be the act of love and service that brings her to Christ later in life. The pope did not give her communion, he performed an act of love and service. God bless him for it!