What are some good locations for scuba-diving within 4 hours flight (under 4000km) from Moscow in early May?

Yeah, Greece or Turkey or Cyprus (Mediterranean) is probably his best bet for near, clear water, but he shouldn't count on it being very warm yet, this early in the year. I worked one season (from mid-March to mid-October) in Larnaca (Cyprus), and on the first dive I did, the water was about 13°C! It warmed up fairly quickly over the next couple of weeks, but didn't go above ~22°C until early/mid- June, so he'll probably need a thickish wetsuit -- 2-piece, 5 mm would be my recommendation (with maybe also a hood).

The northern Red Sea (Egypt) is also a possibility, athough the flight times from Moscow will likely be a little longer than 4 hours. Don't worry about the demonstrations in Cairo -- charter airlines fly in directly to Sharm or Hurghada, so he won't need to go anywhere near the capital. And the package tourists have been avoiding Egypt for months now, so dive prices are relatively low.

Water temperatures in the Red Sea rarely go below about 20°C, even in mid-February (the coldest part of the year), so it will be a much warmer than the Med in early May -- a 5 mm 1-piece 'steamer' may be enough (although a thin shortie on top might also be useful). And the air temps will be a lot higher between dives, so it won't 'feel' as cold.

That one is a clean one, another guy has a dogs and cat that scuba dive. for the reason that there isn't any thank you to regulate the dogs or cat air/oxygen ranges interior the blood and how long it takes the animal to burn off the tiers, then it must be fairly risky. i might say they might get the benz only like we are able to, or worse.

Maybe Greece