No, no, that is, he should go for istikhbara. He should ask Allah for guidance. Why not? Why must he use the wrong terminology and go the wrong way? He must simply beg from Allah to guide him. Hazrat Masih Maud a.s. has mentioned this istikhara in such situations where you have no hatred of him, no animosity towards him and you are in a state where you are neutral and honest about it.
Then he says, if you go for istikhara and say, O Allah, if Ahmadiyyat is right, is truth, then guide me to this. There the words are used, guide me to this. So, if you pray in those words which Hazrat Masih Maud a.s. has used, this istikhara would acquire a dual character, both of istikhara and istikhbara. And those words are the words of Hazrat Masih Maud a.s. and they are the best words in which one can pray in such a situation. So, instead of just suggesting a prayer, you can borrow those words from the Hazrat Musleh Maud a.s. book and guide your friend to that.