You can't assume you know what's going on. Find a compassionate way to ask. Don't start sentences with "you" -- it will put her on the defensive. So rather than "You've been different lately," say "I notice you're more tearful, more angry. Is this something you want to talk about?" If so, tell her how you feel. "I feel afraid for you. I'm wondering whether there's something you're dealing with."
Instead of asking if you can do anything, suggest what it might be. "Can I take a walk with you? Take you out for a meal?" Try to lighten her load: "Can I pick up your kids? Walk your dog?" If you make it open-ended -- "Is there anything I can do?" -- high-functioning people who are struggling may say no, because they don't think they need help or don't know how to accept it.
Of course, there's some "help" they don't need. Rousing statements -- "Don't let yourself get upset about this," "It's not a big deal; put things into perspective" -- are just frustrating. Shoulds -- "You should try harder," "You should be over this by now" -- absolutely do not work; telling people how they should feel rarely makes them feel better. Most people are trying their best to do so -- the reason they can't is because they haven't yet figured out how. That's not a failure on their part. If someone is struggling emotionally, we seem to believe she's not a strong person. But the two are unrelated. Completely.
Anything you can say to remind her of that is good. As is conveying that she no longer has to live with this level of distress. Say, "I care about you, about what happens to you; I want to be here in a way that's helpful." People with mental illness have this fear: If they knew how bad I am, they wouldn't want to be close to me. However you can, communicate that you understand that emotional issues are part of the human condition, that you don't think any less of her. That's what's truly helpful.