The friend that they are to you just might be the friend that they need you to be. L.E.A.R.N. the person for who they are...
Listen. Listen to how (s)he talks when (s)he talks about certain things. Listen to the passion of her/his interests. Listen to her/his heartbreak. Listen to her/his present mindset and if (s)he may want to change that mindset.
Engage. Engage in the conversation willfully, don't just listen to respond, listen to observe. Engage in her/his life events and everyday life. Friendships may have started on one common ground, but the bond is built on foundational mutual support which can only be done by engaging in each other.
Advise. It is not all the time that your friend may want advice, but when warranted, advise. Advise them based on your own life experiences. Advise them on the observations and patterns you have seen.
Recognize. Recognize her/his patterns. Recognize her/his quirks. Show your friend through listening and engaging that you are paying attention and want to further the bond. Recognize that depresses her/him. Recognize what makes her/him smile and laugh and use those things in times of depression.
Notice. Recognize and notice are somewhat one in the same, but you may notice something and not recognize it to the person right away. Any and everything that anybody does can be noticed.
This doesn't necessarily have to apply just to friendships. It can be applied to relationships, workplace acquaintanceships, parent/child bond, etc. Just take the time to L.E.A.R.N. the person you are currently connected to.