If it's a man, don't worry about jeopardizing the relationship.
At the very least he will be flattered and you will boost his
ego.
Just tell them. If both of you are mature, then you should be
able to avoid mixing business and fun. When you're at work, work.
Afterwards, go have a blast. That's not to say that you can't throw
in an occasional nice gesture or word at work though.