Immediately after he starts following you, go to the crowd and move in the crowd until you lose him. The whole thing is rather poorly designed, so it might take a couple of tries, but you know when you've done it.
Ok, that was my original strategy. I got it to work eventually, now I'm afraid I'm softlocked in the vent. I can jump up and down but I can't get out.
Another case of missing input from the game. use Rooter's booster
They sort of hint at it - the woman does say Ace is on one of the upper floors.
