On what basis can a 3rd country legally forbid a foreigner from leaving the country if there is no accusation of crime?

My girlfriend, who is a Russian citizen and resident in Poland, was traveling through Belarus in order to return to Poland. However she was not allowed to leave Belarus when trying to cross the border into Poland; Belarusian border agents claimed she would need a valid reason to leave the country. For example, they asked for a work contract from Poland translated into Russian.

To me, this seems highly illegal, but I have no knowledge of international law.
Is it ok in any sense? Should she complain to the Russian or Polish embassy?