My friend's game shelf went from a sad little stack to a full wall unit in about six months, and it's all because of a local swap meet. | Board Game Geeks - Opinions Matter
Swap meets are great for finding deals, but that kind of growth in six months sounds more like a collecting spree. A full wall unit is a serious amount of games. Maybe your friend just caught the bug hard and found a good source. It happens fast when you get into it.
Honestly, that kind of growth makes me think of my cousin. He got into vinyl the same way, just a couple records then a whole shelf system out of nowhere. Some hobbies just flip a switch in people.