My friend is leaving Moscow for Serbia soon. They use Serbian airplanes which don't have any restrictions put on them.

Serbia does not have any flight restrictions - neither in EU and neither in Russia and Belarus. So they can fly between Moscow and Belgrade...

"Air Serbia Doubles Moscow Frequencies To Meet Demand"

https://simpleflying.com/air-serbia-doubles-moscow-frequenci...

Direct, right? Air Serbia is presumably based in Serbia, not Russia.

Just check flightradar24. I see one from Moscow to Belgrade flying now over Poland.