Hi concerned friend, I would definitely be concerned too.
But it is good that you're thinking about what to do about it/you're not just turning your back on it/the way s/he's feeling.
Not to completely scare you that they may end it all, some people might feel that way but not actually go ahead with it. But for them to be feeling so bad.........then I'd say they definitely need someone to talk to/be there for them/support them regardless.
Now you don't even have to necessarily let them know that you've seen what you've seen/that you know............but maybe find a time you're alone together (with plenty of time to talk) and say that you've seen that they "aren't happy/that they're struggling" (however you want to describe it), and try to encourage them to tell you about it/the way they're feeling/what might be wrong...........and let them know that you're going to be there for them/that you want to be there for them.........and let them talk to you, it might be tempting to jump right in there and try to "make everything right" but, you know, let them talk and work with them on what might make things a little better for them.
And good on you for caring enough to be asking us what you should do.

Alison