Have you told the organisers that he's not going to like this?
You should at least discuss it with them.
I hate birthdays, surprises, parties and that sort of thing. If anyone did that to me, I'd be really upset.
It would NOT be a nice surprise. I might even walk out - it would depend on how I felt at the time.
So be prepared - if you do warn your co-worker he might stay away from work that day, or he might leave early.
And even if you don't warn him, he might walk out the moment they 'surprise' him.
Personally, I'd want to be warned.