Then Company A can protest the claim. If the employee has worked for Company B long enough, he will probably be granted benefits, and such benefits WILL be based on all wages earned from both employers in the base period. Generally speaking, if the employee earned 75% of his total wages from Company A, then 75% of his benefits will be charged to Company A. However, Company A can request a "relief from charges" which, if granted, means Company A's UI rate will not be otherwise affected.
Honestly, if I can say so again, you're REALLY overthinking this. If your employee is causing problems at work, then you either tell him to knock it off (if you think he's salveageable) or fire him, if you don't. Is is really worth upsetting the whole apple cart because your UI rate MIGHT go up .1 or .2% (and even that is not a sure thing)? This is insurance after all, it's there in case it's needed. One firing, even if TOTALLY charged to your reserve account, over 14 years, is almost certainly NOT going to result in a rate increase.

