She definitely did change and most of all she seemed to deeply regret it,freeing those slaves proved that...But still making up,I wouldn't say so.
But I still like Isabela though, also like the fact that she's "grey" but her heart is the right place and she definitely has very interesting character development.
Anyway back OT,I think Cullen will certainly disapprove of slavery being a devout Andrastian.
Just revisiting this a sec:
If Fenris couldn't forgive Isabela for a past she clearly regrets and moved on from, he'd be a hypocritical piece of arse considering he massacred an entire village of people just because he was told to. I understand why he did it, and the mentality he was under at the time, but he clearly remembers it and regrets it and doesn't just hand wave the event. So I would hope he'd be able to understand that Isabela regrets and has changed from her own past actions.
Besides, someone made a confession about that and Mr. Gaider replied to it about forgiving people for their past and stuffs.