Is one who provides services to students for studying in Russia sinful, fearing for them to fall into temptations or change their religion, and is the money earned from that unlawful, despite the presence of Muslims and mosques in Russia?

If the service provided to students is permissible in itself, such as assistance in securing accommodation or registration, then you bear no sin for the transgressions they commit, because you did not aid them in those transgressions, but rather you aided them in a permissible matter. Allah Almighty says: "Every soul, for what it has earned, will be [held] in pledge." However, if you know that the student will use the place for disobedience, then it is not permissible for you to help him.