I doubt that the page ownership changed by clicking on just a link! Imagine this scenario. Someone makes a website, everyone who enters on that website will be redirected to a URL that will hijack their Facebook page, if they are logged in. Report to Facebook, if that's the case, but I doubt he can do something, since he transferred the ownership.
It's not that simple, but that's exactly what phishing attacks do.
From the screenshots of their conversation, he sent him a link to join their business group and authorized them to use his assets by the time I will check he has downgraded him to employee status and take control of all pages.Just report to facebook.
And how come this happen..?
That's not phishing, as you explained it happened! Phishing is when someone enters username/password on a fake website.
Someone makes a website, everyone who enters on that website will be redirected to a URL that will hijack their Facebook page
AFAIK there's no way to compromise an account by just clicking a link.