A bit harsh Wycombe - UK regs for UK flightdecks would have kept him out on a UK aircraft, but maybe the Belarussian regs are different, in which case if the crew were happy then what's the problem? He may be like a significant number of aircrew on here seem to be, and think that the rule is an ass (which isn't to suggest they would disobey it where it applied of course).
Most likely is that the Captain heard that a fellow captain (plus filmcrew were aboard, so invited him up). Most interesting for me given the normal language of the air (though I wasn't sure which crew member was being subtitled) was that one of the guys was talking in a different language (sorry, not sure if it was Russian, Belarussian, but not English) and someone else was translating for him.
Back on topic - I much prefer Airport on BBC1 which has been showing a new series recently. There is more variety in stories, and the curiosity of what dangerous species the animal welfare guy will be wrestling with this week. He's either brave or daft, and must have the constitution of an ox not to heave up at some of the vile smelling things he uncovers!