I think that a mortal sin has already taken place.
You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart. If your right eye causes you to sin, pluck it out and throw it away; it is better that you lose one of your members than that your whole body be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away; it is better that you lose one of your members than that your whole body go into hell.
If looking at someone lustfully is akin to committing adultery with them, then I can only imagine that planning to commit actual adultery would as well, even if one backs out at the last minute. As the quoted Scripture states, adultery is grave matter- why else would the sin be connected to hell? And the man (as a Catholic in a state of grace) has full knowledge and has given full consent- even if he backs out at the last minute. This would meet all three criteria for mortal sin.