What narrative reason is there for Lena to indulge Jack? What does Jack offer that can't be found elsewhere in the story?
Well we don't know yet, as we don't know what Jack's ending from GGGB is yet. Perhaps he's the club owner, or a prospective buyer of the club. In which case he could give Lena a chance at dancing.
Perhaps he's just a VIP because of his time on that reality TV show. In which case he can be a one and done hook-up for Lena, in a similar fashion to bad Jess and Ian.
There is potential narrative elements to what Jack can bring to the table. Although I still stand by Jack's brother being a far more interesting cameo.