Hi Levi @Caleb_Levi
Wonderful question. I live in a Muslim country and deal with a lot of Muslims. The interesting thing about a Muslim is in the undying believe never to question their Allah and whatever happens, is by his will. If things do not happen as per the first time, they assume it is by their Allah will. That is why they always say InnshaAllah. - By God Grace.
When we look at their religion, it is based on the belief that everything is by Allah will and you are not there to question it. Firstly, it is interesting that he said Allah told him to take the leaves. Does Allah actually talk to him? In Islam, Allah talks to nobody. He uses angels to mediate on his behalf. Even their Quran was sent by the Angel Gabriel and not Allah.
But rather than question his beliefs, try applying logic and ask him about why not visit a doctor. If God is sovereign, doctors exist to help by Allah decree. To challenge a Muslim about his belief at the beginning might be a wrong thing to do but showing compassion for him first, helps.
Tell him that his Allah created all and doctors exist by his will. So rather than listen to voices because it may be the devil, why not go to a doctor. Show caring and love to his predicament and over time he will look to your wisdom. That relationship matters and in the beginning, become friends and over time with the Holy Spirit, he will begin to ask you more.
To challenge a Muslim about his religion is to challenge his intrinsic value. Their identity is anchored in their religion. You have to be loving and passionate.