I was wondering what are some things I can say to my friend dealing with depression?

Remember to be supportive of their condition. Giving advice, or trying to fix them is not necessary, but being there for them and making sure that they feel wanted. You can try to motivate them when they’re having a bad day, for example if they’re isolating, try to get them out with you to lunch or another outing that may try to cheer them up. If you’re unsure, I’d honestly ask the person as everyone has different needs when dealing with mental health issues. Reguardless, as long as you’re being a good friend to them, there shouldn’t be any problems in what you’re doing. Keep up the good work and keep being a kind person :)