Calling him an idiot is not fair at all.

To go to any country from Russia, you're supposed to cross the Russian border with passport control. The Russian guards would probably detain him there.

But Russian-Belarus border is an exception, it's kind of transparent, like a border between two EU countries. There could be some occasional checks, but more often than not you just cross it without being asked for the documents.

Once in Belarus, he could try and cross, say, Belarus-Polish border, less risk.

His plan didn't pan out, but it was probably the best plan he could have had.