Ask your friend what would be useful. Ask your friend about a "rainy day action plan" - this would be a list of things for you to look out for, and a list of small things your friend could do to let you know things are particularly tough for them.

For example, some people find it too hard to say "Things are tough. I need help" but they would be able to wear a particular hoody and listen to certain music. You'd know to look out for those things, and be able to ask how things are going.

When your pal is having a tough time you might want to allow them to have a week off, and then help them get back into society. They may need encouragement to cook food (or eat good food), or get exercise, or leave the house, or what not. Don't do all this stuff for them, but do help them get it done.

Some people may be at risk of suicide. Saying "Are you having thoughts of ending your life" is acceptable; if they are you need to get them professional help.

There are almost certainly good groups in your country that can provide more help and advice.

This is just advice, I have no professional knowledge etc etc. Good Luck!! It's a tricky thing to deal with.