While only a professional can give a certain diagnosis, your close relationship with that person makes you very sensitive to their moods, and there is a good chance they need some form of help.
Although this person may not respond positively to your attempt to help initially, there are a number of things you can do to start the healing process. In summary, you need to talk with the person in a certain way, and also educate yourself about the different types of depression. The more you know, the more you can help your friend or family member.
The article below provides a well-balanced strategy to begin to help: 9 ways to help a friend or family member with depression
For professionals to consider a diagnosis of a serious depressive disorder, symptoms need to be present every day for at least two weeks. There are two main categories of depression: Dysthymia, and Major depression. Both are serious conditions, but treatment varies depending on which disorder a person has. WebMD provides clear definitions of Dysthymia and Major depression.