(Disclamer: this is a fairly sensitive topic, and I'm not a lawyer. If you have any doubts, seek legal help.)
In theory, the Russian constitution states that Russian citizens always have the right to return to Russia. However, it's best to follow the standard procedures to avoid complications at the border or during the flight check-in. This depends on whether it's the internal or the foreign passport that has expired.
If your friend has an expired internal Russian passport he or she will need to apply for a new passport upon arrival, otherwise they'll face a penalty of up to 5000 RUB (~80 USD). The internal passport is normally not verified upon your departure or arrival so there should be absolutely no problems going back to Russia with an expired internal passport.
References: Code of the Russian Federation on Administrative Offenses, pravoved.ru.
If your friend has an expired foreign Russian passport, they need to get in touch with the nearest embassy or consulate and (likely) fill in the re-entry application. It should take 2 days for the re-entry certificate to be issued.
References: rg.ru, garant.ru.