Advice him/her to seek help from the professionals. It would be better if he/she would seek help to professionals to have a proper guidance on what his/her problems and get some advice that would lighten his/her burden. If it would not work, help him/her to confide to someone he/she trusts. If he/she sees that someone loves and care for him/her, it might somehow alleviate the suicidal thoughts and plans running in his/her head.