Coming up is a "yahrzeit" for one of my best friends, who died only a few years ago. It's not a real yahrzeit, of course, because we're not related, but I wish there were something I could do to commemorate his life. Any thoughts on how to mourn friends?
I suppose I should focus on comforting his mother and family, and be content that I don't yet have an actual yahrzeit to observe, because in time they will all come.
Loss is loss. Just because you may not be halachically required to go through formal mourning for someone doesn't mean that you can't mourn them in whatever ways seem suitable to you.
One way is to give tzedakah in his memory. To do some positive action on his behalf is a great way to honor him.
Another way is to learn Torah in his memory. You could host a study session in his name-- invite a bunch of people to learn together, either as a group or various texts in chavruta; or you might commit yourself to learning a whole section of text in his name-- a chapter of Mishnah or Gemara, a book of Tanach, so forth.
Another way might be to host a kiddush or something in his memory.
And in addition, there is nothing that says you can't light a yahrzeit candle for him, recite "El Malei Rachamim" or even kaddish (it's not forbidden to say it for someone you're not required to mourn for, it just isn't mandated). You might also recite some tehillim for him-- Ps. 23, maybe Ps. 91, Ps. 121, and so forth.
Do the things you feel will help you honor his memory and make you feel right.
Hate it when I do that ...
