Ask them what it is that's causing them to feel low. Discuss it with them and either offer advice to improve the situation, or try and get them to think more positively about things. If they're suicidal and you really can't think of anything, 1) don't blame yourself. It doesn't have to be your job to talk them around, and 2) If you're in the UK, get them to call the Samaritans (08457909090) or something similar for outside the UK.
It may also be advisable to read through this -
The Suicide Information Package.
Most importantly, take care of yourself. Don't put others infront of you if you're not in a good place yourself. If you need someone to talk to, or some help for your friend, please PM me.
Sarah x