The UK is not the EU, so there could be different requirements on that. For our EU visa application, we submitted both a sponsor letter and bank statements of the applicant. The visa center accepted both without any problem. Having proof of a way to pay outside of Russia can never hurt, since Russian Bank cards no longer work outside of Russia (i.e. my spouse will pay with their American credit card). Also might depend on the relationship. If it's your spouse, should probably sponsor and book hotels in both names, as it could seem a bit odd to go on vacation without your spouse. If it's a fiancé, might not be worth it, as we had to submit proof of relation (marriage certificate). I think the directions said for none-family sponsors, the sponsor letter had to be notarized. Lastly, depends if the Russian applicant can actually afford it on their own. If they don't have a million plus rubles chilling in the bank or a very high salary, should probably add a sponsor letter. This is just my opinion though, so take it with a grain of salt.