What a beautiful, heartwarming sentiment you intend and as always, I'm here for you.
Send a card, as you planned (it's never too late) and include a particularly meaningful memory you have of their dad. It could be a boating outing or drive that was taken, some advice that came in the nick of time or words you've never forgotten - just take a moment and replay all those years - what sticks out? Often, the most simple and typically 'forgettable' of times are the ones we hold deepest in our hearts.
Also mention at the end of the note that you've sent a donation or are doing something more creative, like bringing a pkg of paper towels or bedding to your local shelter in their dad's honor - or even volunteering some of your time, the most precious of gifts.
Tell them you're going to give them time to heal, but would love to reconnect so you'll be calling in the near future and perhaps can get together (if close enough?) - and of course offer your own contact information with the encouragement for them to reach out "any time, day or night"
And on a personal note to you, I'm sorry for your loss. Keep reaching into your heart for the memories of your friend and use them to get you past the feeling of having lost someone. In those memories, they are always there.