There is a saying within the BDSM crowd I run with ...
"If you make your sub cry...then you've done something wrong."
Never cross that line when the scene becomes way too painful for your sub or slave for that matter..and their crying or begging for you to truely stop...THEN STOP.
I've heard horror srories where guys(wannabe dom/masters) take that as fun and games..it's not.
so in keeping with the topic..I say hell yes she should be able to get out of it if it becomes too much