He's long the 170 put since he said in his original post: "I can exercise the 170s as long, but not the 150s as short."How do you mean 'the other way around'? If he's short the 170 put and long the 150 put then he would need to buy the 170 put and sell the 150 to close.
Yes, the options are illiquid with wide spreads. The Oct 150/Sep 170c diag would cost 70 cts. which isn't much better than closing the 20 pt. put spread for the natural credit of $19 ( - $1.00). Given those two choices, I'd buy the Sep 150c for 40 cts, assuming no generous splitting of B/A's by the MM.