If it was me, I would meet. You two guys were friends and connecting with people is mostly a good thing.
This might very well be an attempt to recruit you to his company, or an attempt and gathering information. However I would not assume a nefarious context. I would assume that he misses a respected colleague. He might also be seeking info on coming back to your company. Who knows!
I might rehearse some lines to say if information or questions hit a bit close to home. He might want to innocently want to bash your boss, but I would ask him not to as you still have to work there.
I would also not advertise this to anyone in your company. Bosses are human and can easily become jealous or threatened.