I find the way you present it surprising. Nowadays, requiring a quarantine period (or at least a negative PCR test) for people coming from a different country where there is risk they could have COVID seems the rational thing to do, for all countries.
The actual implementation could vary from being enforced by having military personnel watch that interned people do not break their curfew to just asking nicely and expecting that people would follow their recommendation.
could an international company enforce a realistic and mandatory quarantine period on their employees?
As mentioned by other answers, there is an agreement between the company and your friend that he will quarantine himself. This would probably imply he would lose the job position if he didn't comply. This is probably a good enough way to enforce it by itself.
However, taking your question in a more strict sense, yes, I think an international company could take stronger measures if so they wished.
Suppose company is incorporating 100 foreign employees, which will arrive to the country on September 1st. The company contracts a hotel to host them for the quarantine period. Other than hotel employees, everyone else is expected to stay on their rooms for two weeks.
Could a guest sneak our of their room and out to the street without hotel security noticing? Perhaps. However, the risk would be high for little reason.
(This could make the argument of a novel, btw)
Could he be physically restricted from leaving? Maybe not. And if he simply stated very firmly that he wished to leave the hotel, they might have no ground to force them to stay. I wouldn't be too surprised if they nicely informed him that in order to leave that place early he shall do so by taking the next plane out of the country, though.
I would expect that in such situation the company would find/provide a place to live (a corporate housing, perhaps) those two weeks, since the new employee wouldn't be in a situation to search lodging by himself (and it would be unwise to have him visiting different places). In his situation, I would also request that the company provides a procedure so that someone does the shopping for him (maybe a courier, being told about a local grocery store supporting online shopping...), so that he can properly follow the quarantine the company is expecting from him.
Plus, obviously, while being those two weeks on payroll (either if he is asked to start working during the quarantine or not, although the company would probably prefer that he does).