Are you still having fun playing with just you and your friend? Cause that is the only reason that I can see to continue playing. If you are having fun, then keep playing. If you aren't, then stop playing.
The amount of money that you put into the game before shouldn't even factor in. If you aren't having fun, then why would you keep investing money in the game? Just quit. You've already gotten your monetary value out of the game anyway. Just think of all the hours of fun that you've had. Spending money on cards is just like spending money on watching a movie. They are both forms of entertainment. At least you get something tangible from cards, though. If you are having fun, then what does it matter if there is only one other person. Maybe you can just enjoy collecting the cards or maybe you can travel once a month to the tournament that is ****her away.
In the end, it should all come down to the question of whether you are still having fun or not.