I've just been reading info about US prohibitions on tourism to Cuba.
Cuba is on the list of State Sponsors of Terrorism. Russian Federation is not. Nor is US at war with Russia.
So the Americans are not in trouble with their own government. Nor, AFAIK, breaching Russian laws, although Russian authorities might not need an official policy to be unfriendly

US financial sanctions on Russia will make it impossible for those persons to pay to get out of Russia. CX also, to the extent it needs to use USD or pass money through a bank that handles USD.

Some consideration will also have to be given the Americans are travelling on a China (Hong Kong)-flagged plane - although maybe Foreign Ministry might "accidentally" overlook that.