Answering such abnormal cases requires a combination of jurisprudence and wisdom. I would say:

First: The default ruling is that a Muslim is not permitted to sell or possess anything prohibited, whether it is food, drink, or any commonly used items. This ruling applies both in Muslim-majority lands and non-Muslim-majority lands.
An exception exists according to the Ḥanafī scholars, who allow selling alcohol or pork to non-Muslims in non-Muslim-majority lands.
However, we must clarify that the default ruling for a Muslim is prohibition from wasting his wealth on things of no Islamically legal value, such as alcohol or pork. Therefore, the Ḥanafī position mainly applies to cases where the Muslim is not directly profiting from selling these items — such as when he works for an entity that sells them and receives his salary or part of it, or when he acts as an agent (franchisee) for a company dealing in these products.
In such situations, the earnings are valid based on the described contracts.

Second: A Muslim is commanded to stay away from places of sin as much as possible, based on Allāh’s words: “And when you see those who engage in offensive talk about Our verses, turn away from them until they engage in a different topic. And if Satan causes you to forget, then do not remain seated after the reminder with the wrongdoing people.” [Al-᾽An῾ām 6:68]
A gathering where alcohol is consumed is a sinful gathering, just like a gambling session or a gathering involving dancing and nudity.
This principle applies in both Muslim-majority and non-Muslim-majority lands because it relates to the Islamically legal accountable individual’s actions, conducts, and choices.
However, this principle may be encountered by the fact that some practical necessities in non-Muslim-majority lands may sometimes impose certain obligations on Muslims by virtue of their work — for example, those working in public transportation services (like Uber drivers) cannot refuse to transport intoxicated passengers/intoxicating items indiscriminately, and they are only allowed a certain number of refusals before their licenses might be revoked.
Similarly, refusing to serve someone because of religious reasons or appearance could sometimes be construed as discrimination.

Accordingly, the Muslim business owner must be aware of the legal regulations surrounding prohibition and permissibility in his restaurant or business environment, yet he is allowed to prohibit the entry of alcohol or those carrying it.

In summary:
Regarding the current situation, since the questioner did not directly sell the alcohol to the customer, he is not responsible for what the customer drinks in terms of the questioner’s earnings.
He has the right to prevent entry through clear prior announcements.
As for what has already happened, he does not need to do anything, and his earnings from selling ḥalāl food remain lawful.

Fatwā issued by Dr. Khālid Naṣr