You would say "I know it's belated, but I hope you had a great birthday". You can buy a birthday card for a belated birthday. And yes, you can say "Happy belated birthday". Then the friend will know that you realize it already happened. Don't put the belatedly at the end, like you did....it is a bit awkward.

Apologize for missing it, and claim you were thinking of them that day. Do not use the word belatedly. It is not wrong to do it, but it is a strange word. It is clunky and even unfriendly. What I mean by that is, you should be informal with your friend, and belated is a very formal word you would use if you mss your bosses birthday... Just say, sorry I did not call on your birthday, I was thinking of you! Hope it was great.