Yes, you should leave him.
Now see, I question whether that's a catch-all correct answer in this instance. Perhaps if you explained your reasoning it would make more sense to me. (Though, truth be told, it was my initial reaction, as well.)
There's just too much unknown.
For starters, it doesn't seem as though any kind of real negotiation took place prior to this, and that would be the time to do such things as agree on STI- and pregnancy-prevention measures, make a plan to ensure open communication, discuss whether or not they both wish this to be an ongoing arrangement, etc. It's never too late to have that conversation, though. She has the right to withdraw consent and create boundaries at any time.
There's not enough information here to lay down a concrete "gtfo," from my perspective.
I feel that it's pretty likely that he probably pushed for it, but it's possible it was a mutual interest. We don't know how long they've been together, or if their relationship is otherwise healthy. We don't know if she's talked to him about her feelings since then, or what, if anything, they discussed leading up to it. She may have entered into it telling him to do whatever he felt like, and then realized after the fact that they should have taken more time in the planning.
We have zero information regarding the circumstances surrounding this situation. Certainly if it were me in her shoes I would dump him and never look back, but I also would have had a serious negotiation prior to agreeing to it in the first place. I think she could stand to TRY to communicate with him about how she feels before salting the earth.
Can you explain your rationale? I'd genuinely like to know what's driving your assessment. I'm rather new to poly relationships and your perspective would be valuable.