Most often these things are best summed up with music.
So here are two offerings: one to a friendship gone (and a very happy upcoming 50th - I wish you nothing but happiness), and one to a friend departed.
I know Alun liked ELO, so this is for him - the guy who was my squad mate, introduced me to my first fiancée, talked me into joining to karate with him, an all-round great guy, friend and power-house of a swimmer, with a hint of mischievous humour running through him.
If he could read this, or could hear me, I would tell him, 'Miss you buddy. So, so very sorry for not being there for you.'