I think it is, at least in some territories namely russia.
But even if it wasn't, what's the point on taking someone else's work and make an unauthorized sequel for it? It would only make you look bad.
It would be one thing if he wanted you to make it but I highly doubt it's the case.
Well for one the game is on Steam. Also on <a href="http://www.koshutin.com/">this site</a> the title is "HAMMERFIGHT(TM)". In this case I'd not try to pass it off as an official sequel. It's up to you really though.