Originally Posted by
MaecDavidMiller
UNLESS she was a citizen of the Russian Federation (not USSR, Russian Federation).
The OP clearly mentioned she is.
If that is the case then you need to address the issue, preferably with a Russian immigration attorney.
There are no legal issues with Russian citizens going abroad for permanent residency.
There is a practice of amending a passport with the rubber-print "residence abroad". In the past, only those who had it might register with a Russian consulate in the country of residence. Nowadays consulates will register any Russian citizen; so that practice is just a rudiment.
Anyway, the OP's mother-in-law has already lived abroad (in Bishkek), so the question itself is ridiculous, especially given the fact that she won't pass immigration in SVO.