Pray for them. Christian people will often pray for or with others as their primary way of helping. This is a great thing to do and mental health problems are just as much a sign of world where all is not "as it should be" as many other issues that we offer to God in prayer. We need to bring our concerns for ourselves and each other before God and seek His guidance.
But it is important to listen to what the person who is unwell or struggling would like prayer for, and not assume that we should be praying for "deliverance from a demon" or even for complete restoration. Mental health problems are complex and relate to our identity and our deepest beliefs. The person who has one is often the expert, so ask them and God what they would like you to pray for.
Many churches have pastoral or prayer teams and these can be an invaluable resource praying with people when they are most in need and praying for people as a part of their regular prayers.