Player A has Mercurial Pretender on the board (copied a 4/4 creature)
Player B attacks with Typhoid Rats
Player A blocks Typhoid Rats with Mercurial Pretender
Player A then pays 4 mana to "Return this creature to his owner's hand"
Player A returns Mercurial Pretender to his hand, then says Typhoid Rats dies and goes to graveyard.
Player B thinks that player A cannot both successfully block and return Mercurial Pretender to his hand, considering Typhoid Rats Deathtouch ability.
Who is right?