What to know Muslims have been taught that their highest goal in life should be to submit to God, not to know him. In Islam, God is transcendent: human beings can't know him or have relationship to him. Followers of Jesus also affirm that God is transcendent (beyond knowing)...but we also believe that God has the power to reveal to us that which we can't grasp on our own (Ephesians 3, 1 Corinthians 2.) For many Muslims, dreams hold a special significance: They're seen as a place where a human can receive a message from God (Sura 8:43). What to say Ask your friend to share their dream. If the dream includes Biblical imagery, share passages that can help them understand its symbols and interpretation. Many of our Muslim friends have dreams that align directly with Scripture, despite having never read its pages before!
You might also encourage your friend by inviting them to read the accounts of other people in the holy Taurat and Injeel who have received dreams from God. Pray and ask for God's leading of which passage to read. Some recommended options are: Yacob (Jacob), Yusuf (Joseph), Daniyal (Daniel), and Yusuf the husband of Maryam (Joseph, husband of Mary). Joel 2 is also a powerful passage that names that both men and women will receive dreams and visions. (Acts 2:14-21 places this prophecy as not only in the distant future, but having already begun in the days after Christ.)
Last, if God leads, ask your friend how they feel we should discern if a dream or a vision is from God. If they're willing, ask them to expand this topic into discernment about prophets as well. How can we identify what's from God, and what isn't? Listen well, and read Jeremiah 23:25 and 29:8-9, together with Deuteronomy 18:15-22 and Matthew 7. Although some people exploit the reality of dreams and visions in order to deceive, God has provided a clear way for us to recognize a true prophet.