What to do if a friend is thinking about sucide?

Ok so my friend is thinking bout suicide not because she wants to die but because of a stupid pact she made with her boyfriend. He died like a year ago and they promised each other if she didn't find somebody she loved as much as him she would die in a years time. so she found someone but it didn't work out and a year is up in June and im really afraid she'll do it. I can't tell an adult because she will deny it. What should i do?