Got a co-worker looking to get more serious about riding.

He's 6' 4", older gentleman. He's still not sure if he wants Hybrid or MTB. He will likely be doing mostly greenway riding, but is also interested in the "less foot traffic" aspect of trail riding. I suggested he get a 29er hardtail MTB if he is at all considering actual trail riding. 

His budget maxes out at $600, though he would probably be more comfortable with $400-500 for a used bike.

Whatcha got?