3mo tourist at entry with unlimited border runs in Th.
This is definitely not correct.
However, Russia belongs to a group of 6 countries which has intergovernmental agreements on visa
waiver in place. This gives a Russian citizen the right to enter
Thailand for a max. of 30 days. Vice versa, a Thai citizen can enter Russia on the very same basis. A Russian citizen is also eligible for visa-on-arrival but this is currently not being processed for more than 30 days (it used to be more in the past) - so it has no advantage.
However, this does not have anything to do with border runs or a 3-months visa-free stay, the latter currently only being available to Argentina, Brazil, Chile and Peru.
As
@JamesDonkey already mentioned, all this is irrelevant since it has nothing to do with legal residency. A fool who in today's compliance world still relies on border runs. In most cases this person will not even be able to produce a proper Tax ID.
If Thailand is the destination of choice then a Russian citizen should opt for a Thai Elite Visa (very easy to get).
In case of the person meeting the age requirement, the so-called "Retirement Visa" might be a cheaper alternative. It is basically just an "Extension of Stay Based on Retirement” and comes as an extension of a Non-Immigrant O Visa or Non-Immigrant OA Visa.
N.B.: If the person meets the age requirement, the best program is available in the Philippines. Clear papers, life-time residency, fast processing, ideal for
crypto transactions since many banks are actively promoting it.