He has a residence permit that allows him to get past Russian exit controls and travel abroad, so I suppose he can get on a plane if he wishes. Getting off will be a problem, yes. He'd have to arrange something with Ecuador, Venezuela, Cuba, or the like.
Airlines generally won't let you board unless you have the necessary documents to enter the destination country (e.g. a valid passport, a visa if required). In fact, countries will fine airlines for bringing passengers without the correct documents.
As far as my experience goes, the only thing that gets checked in russian airport (for resident) is your passport. Passport control doesn't know where you are heading nor they care, nobody else checks it.
Maybe this is different matter for some flights (for example, to Israel or the USA), where the airline does additional checks on you, but you fly out to Europe and most other destinations unchecked (despite needing a visa to enter).
I've travelled quite a bit (including Russia -> EU), and I've never been asked that.
Maybe they do a fast peek at your passport when letting you to board in the very end, but they can only check if some visa is physically there, since it takes them around 5 sec.
