It is very brave for you to be taking the first step in recognizing your friend may need help! Something that is recommended for lots of people in your position is that you make sure to let them know that they are supported. Saying kind words like: "I care for you a lot, and I noticed that you seem to be not feeling your best, but I am here on your good and bad days." can make a big difference to someone who may be struggling with depression.

There is also a hotlines page to speak with professionals if things become serious and out of control. However, you are always welcome to message here from people who have been in your position. :)