Upvote:3
According to the Russian Constitution, Russian Citizens cannot be stopped from returning to Russia, so, even if your friend does not have the international passport, (s)he can return back to Russia with internal passport.
However, such situation will lead to administrative fee about a 3000 RUR during border control, and it may take up to 2 hours to go through all that procedure.
What your friend can do is to contact the nearest Russian Consulate or Embassy and get the special document for such situations called Свидетельство на возвращение в Россию
(Certificate for return to Russia), which is being used exactly for such situations. After getting this document one should use that document for check-in within carrier. Please note that such document cannot be used to go outside the Russia.
Documents needed for this certificate is:
If your friend cannot get the certificate in time, (s)he should notify the Wiss that the passport for check-in will be the internal. It's not look like that they need to do something special in that case.