That's what the subjunctive is for. If A had said “If Jon came to the party, Mary would be happy”, ...
The same thing can still happen with a subjunctive conditional, though.
A: If John came to the party, Mary would be happy. (So we could make Mary happy by making John come to the party.) B: But John isn't going to the party, no matter what we do. (So your argument is invalid.)
Also, pace George R. R. Martin, the name is still spelled John. Sorry, no offense, I just couldn't resist. :)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.