Hi Sara. That is very kind and considerate of you for asking how to help your friend. He is fortunate to have you.
I'd like to seek advice regarding things I may be able to say and coping mechanisms I may be able to suggest to bring him up a little when he's feeling his lowest.
Not knowing all about your friend, I will share what helps me when I feel that way. I appreciate when my loved ones let me dictate the energy of our interactions. Being a quiet, introverted type, I like quiet meaningful conversations. I like to share what is on my mind and for my listening ally to just "attend" to me - acknowledge what I say without judgement. I also appreciate them reminding me about the good parts of my life - without making me feel guilty, like I "should" be feeling happy and not sad. I also like it when they remind me of my strengths - the parts about me that I cherish the most. Because when I'm down, I forget about those parts. All I can see are the sad emotions and state of despair.
Sometimes an invitation/encouragement to go for a simple walk helps a lot. It gets me out of that state and gets me re-focussed on the beautiful parts of this world.
A lot of patience is helpful too. That can be hard sometimes, as a caregiver, because it might be hard for the listener to understand why or how a depressed/suicidal person keeps looping back into their low moods. So not casting up how I was feeling good last week, what's wrong with me, why did I suddenly plummet - that kind of talk, although well intended, will just make me feel shittier about myself and guilty.
And remember self care for you too. Supporting a person who tends to slump into low moods can be tiring and taxing to you too. Remember all you can do is show unconditional support and love, but ultimately it is the other person's responsibility to make changes. We all need a hand up and out of the pit of despair and depression - but we got to do the climbing ourselves.
I hope your friend finds a way to see the light in life and to feel your support.