The title and framing of this question seem to question whether this event was any of the employer's business, given that the event was after hours and outside work premises.
nvoigt has already given an answer explaining that in Louisiana and other "at-will" jurisdictions, the employer can make almost anything their business.
It might be worth supplementing that answer by noting that even without that at-will context, it is generally not safe to assume that an event of this kind would be outside the employer's remit.
Even when not organised by the employer, events like this still have a significant connection to the workplace and what happens at such events can impair co-worker relations. In light of this, workplace law in non-at-will regions will sometimes recognise such events as an extension of the workplace and subject to employer disciplinary processes.
For instance, take the Australian case Drake & Bird v BHP Coal Pty Ltd [2019] FWC 7444. Drake was fired by BHP for egregious behaviour while drunk at a work party, which BHP considered a breach of its employee code of conduct.
As with most workers in Australia, Drake was covered by legal protection against "unfair dismissal". Although he didn't contest these allegations, he argued that his dismissal was unfair because the event wasn't an official BHP party, or that he hadn't known that it was. (He claimed to have thought it was just an unofficial gathering of employees.)
The commissioner hearing the case found that Drake probably did know BHP had sponsored the event, but then went on to state that even if it had just been an unofficial gathering of employees, it might be considered "related to work" and subject to BHP's disciplinary process. (Paragraph 115 at the above link.)
In your case, the event was organised by the employee's boss. That kind of thing increases the chance that it would be considered a work-related event where the employer had a say in employee conduct.
"Excessive drinking" on its own, without any accompanying bad behaviour, is much less likely to be considered justifiable grounds for disciplinary action. But one shouldn't rely solely on the unofficial/off-the-clock nature of the event as cover against disciplinary action here.