If the cops come by, and ask you where your friend stashed the murder weapon, and you refuse to talk to them - probably defensible

Absolutely defensible. You are under no obligation to talk to law enforcement and as a matter of fact it's highly advisable that you don't say anything at all until you have a competent lawyer present.

I'm not a lawyer, but Ken White of Popehat is and he has a lot to say on the subject:

https://www.popehat.com/tag/shut-up/

Specific insight why you should really shut the fuck up until competent counsel is present is provided in this post:

https://www.popehat.com/2011/03/18/just-a-friendly-reminder-...

Edited to add this other relevant link:

https://www.popehat.com/2007/10/26/shutupshutupshutupshutup/