That doesn't track with what Maeve says in Cold Days on Demonreach.
It sounds like Maeve and Beside are picking their next target right then and there. She's not talking like Justine is already infected.
We have to re-evaluate Justine's actions ever since White Night at least. I re-read Even Hand yesterday and it totally makes sense if you assume that Nemesis made her involve Marcone in the first place. She got to check him out. Marcone was in possession of Namshiel's coin, who is probably the Nemfected Fallen. If Marcone had lost, the coin would have been available again, maybe for her to retrieve it and look for a suitable host. In case he won, as he did, a minor Fomor lord was lost, but Marcone would have been motivated to pick up the coin himself. I suppose that is what happened. At some point after Even Hand he decided that he needed a power-up very badly.
Cold Days is tricky. I always found it odd that Thomas insisted on taking her along in the first place. She would have been much safer at Castle Raith ...
At the show- down, she neither helped Maeve nor team Harry. She just observed. As usual imo. Was she there to make sure that Maeve was not doing anything stupid? As Maeve then did?