Is she the type of person who would have a problem with you going on a date with your ex? Because that's what this is, essentially. Unless you're a really good friend with her ex, this isn't what girls do, when they're in a relationship that they actually value.
Is her ex a guy who showered her with gifts and paid for everything?
In your case, it's a bit too late to voice your disapproval of her going to the cinema with her ex, as you told her you're cool with it, so you'd be going back on your words. Just make sure that kind of disrespect doesn't happen again.