You’re not wrong, but I’ve also worked at companies that successfully contested unemployment claims. It can depend by state, but “it was entirely this person’s fault” is a bad start. Employers win about 30% of contested claims, and then about 15-20% of appeals (#1 cause for an employer losing a contested claim or an appeal appears to be withdrawing or not showing up for it). (Some numbers)
And the main reason employers lose when they show up is lack of preparation. In a case like the above, if they can show a policy (preferably one signed by her) that directly forbids her onlyfans account, they probably have a pretty good case to shut her down.
That said, they’re very unlikely to waste their time and money to fight it. Ultimately (as my current employer’s HR put it) “it’s just a cost of doing business” and a waste of money to pursue.