Is a Russian passport holder with a 45-day entry stamp from Malaysia eligible for an extension in Thailand?

Not eligible for an extension as they entered under the visa waiver based on a bilateral agreement.

Some offices may offer a “7 day extension” but this is not what it is. It’s a denial of an application for an extension and after denial they give you 7 days to leave.

This is not a good stamp to have for future entries here or possibly in other countries too.

Easiest way would be to “border bounce” to another country by land or air and return for another 45 days.