True, I agree that she's doing the right thing. But she is literally laying down the law to affect his behaviour. Which is fine. You keep seeing her, I will leave. Her leaving is a promised "punishment" for his "crime". The only reason you would give that ultimatum is to affect his behaviour. So in the opening line "I'm not telling you what to do" she most certainly is.

I think they have an implied social contract about behaviour. "We don't sleep around or have other intimate partners". He is close to violating that contract and she wants him to NOT do that so she is outlining a new rule. "If you want to continue in this relationship, you must stop that other relationship".

"free to choose" is a hugely loaded phrase. I suppose I am free to choose to go on a shooting spree, but I'll certainly be punished for it. So, am I really free to choose that option? In theory, yes, but in reality, no.

This is just a fun little "word usage" argument which has little bearing on the true content above. :)

I still support her move on this.