Can you ask Allah for forgiveness for another person?

If there is a person who did a serious sin (like suicide) because of some mental illness. So basically he/she is already dead. So can't ask for forgiveness. But you don't want to see that person in Hellfire. So can Allah forgive Him/Her because of me praying for Him/Her for forgiveness and Will Allah accept that forgiveness for Him/Her forgive that person and let that person enter in Heaven?