He could be in trouble for working on a visit visa. He and/or the employer could be fined, he could be jailed and/or deported.
However, he shouldn't be subject to a
UAE labour ban if he leaves his current employer for another one, assuming he doesn't have a labour permit or residence visa, and isn't here on an employment entry visa.
I suppose if he had signed a letter or employment agreement with the current employer, then they could try to make a civil case against him and sue him for financial compensation if he left abruptly, but I think that would be unlikely given that then the employer might have problems because they employed him without the correct documentation.
If he hasn't signed any agreement, and has not handed in his passport or any other papers for processing, then I don't see how the current employer can make a case against him easily. A verbal contract is just as binding as a written contract, however it is much more difficult to prove.
Otherwise, if the employer has some
wasta with the immigration department or labour department, they might be able to cause problems for him.