Unless the UK government where OP lives prohibits playing on crypto gambling platforms that are unlicensed or are not licensed by the UK Gambling Commission, the player has every right to use his crypto to make bets. Taxable events occur when the crypto is exchanged for other digital assets or fiat currencies, which wasn't the case here. I seriously doubt he was breaking any laws. But I also don't think UK authorities can investigate a Russian citizen (if the owner is Russian) who operates an anonymous crypto casino. That's out of their jurisdiction.