Ask a very saintly devout Catholic to pray for you.
A prayer from an apostate is not as efficacious as a prayer from a devout Catholic who strives to be holy. It also seems that in specifically asking an apostate to pray for you, you are being an accessory to apostasy if you know they will be praying in an inappropriate manner. Now, I speak of a Mormon who knows the Truth about the Catholic Faith and obstinately rejects it and has a tendency to pray in an apostate fashion. In the case of an invincibly ignorant Mormon or even a Mormon who is more open to the Catholic Christian method of praying, I would not see anything wrong with that so long as you ask them to pray to the one God in which we hold in common. But, again, it is not as efficacious as that of a Saint or devout Catholic who strives to be holy.