If a friend has lost someone close to them — a parent, a grandparent, a friend, whatever — put a yearly reminder in your calendar of that day.
Then, when it comes (and it’s appropriate based on this friend’s personality) just send a text — “Hey, I know this is a hard day. I love you and I’m here to talk or eat ice cream with or whatever you need.”
The chances of you “reminding” them of this are pretty slim. They have not forgotten the day their friend died. In fact, they are probably thinking a LOT about it, but to everyone else, it’s just a normal day.