Great guide is already up, this is just in case anyone prefers a video to text.
Just follow the video and you will know exactly where he is!
USUALLY, he will be in the location shown in the video, but what you have to do is...
Right after you tell Osypenkos GirlFriend that hes upstairs when you get into the party, thats when you make your move to Bogdan but be aware, he can start walking around downstairs..
Doesn't make TOO much of a difference but he has a bodyguard with him, so my advice, get to Bogdan and pickpocket him ASAP.