How dangerous is it to work temporarily under the table without a visa?

Hey, looking for advice here -- any would help! My boyfriend has been trying to get a US work visa for a while now, but it's really hard finding jobs from overseas. An option would be for him to come here and work under the table while he looks for a job that would sponsor him and get him a work visa. Not asking about tax stuff, I am wondering if anyone has experience doing this or knows what would happen if somebody/government found out he was working without a visa? This would only be temporary until he got a visa, under a year.

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