You've encountered one of the toughest things a group admin can encounter: a situation that has no clear answer.

I can't offer advice, but here's what I think. My experience says it is possible for even just one person to poison a group. Generally they do such things deliberately, and it is obvious that is the case, so kicking them out is a relatively simple decision. But when they are damaging the group unintentionally, things get harder to handle, as you well know.

If you're not at the point of no return, you might approach the individual in question and ask them to moderate their own participation. Perhaps they can make the same points in a less antagonistic manner? It is generally possible to disagree respectfully, and making it clear that they are doing more harm than good - perhaps they are even undermining the very points they want to make in the process of making them - might induce a behavioral change.

Or not, in which case you'll have to assess the value of having the new and different voice in the group vs. the loss of cohesion and community, as well as the possible long term damage done to the group.

I suspect I know which group you are referring to, and I know how much value I found in the group 7 years ago. You will have to assess that now and determine the proper way forward if it comes to that. I hope you have a few other moderators you can talk it over with.

Good luck!