If she were to walk from the bridge, to Topkapi, then to Serkeci, it's about a 35-minute walk. She had over an hour to get from the bridge to Serkeci. Do we know know she was in Cankurtan 'just after that', or was it later...?
http://goo.gl/maps/tDIAi
I wonder if she went to Topkapi, then looped back around towards the bridge, looking for Tarkan. But then ran into this other person along the way. Perhaps he followed her to Serkeci, or they went there to grab something to eat/coffee (it was lunchtime after all). Then he (if it's the other landlord) told her he wanted to show off his rental units in comparison to where she was staying, etc, and talked her into going back to his place with him. (I have a hard time believing that she may have gone around having sex with multiple men. Some women do, for sure, but I think she was just dipping her toes in the water on this trip with Tarkan - and maybe Ammer). The rest is history. Viable theory?
Heck, maybe when she didn't meet up with Tarkan that day (IF she didn't meet up with him), she sought out the man with the business card, knowing where to find him that day, instead. It *was* her last day - maybe she was looking for a friendly face, someone just to say 'bye' to.