Xanax works on the GABA receptors and if you want to have less withdrawal after getting of it you don't smoke weed or drink booze because both of those items actually inhibit GABA. What he needs is another drug to replace one he quit. Thats part of how drug addiction works as you need to replace voids.
Some people may find pot helps xanax withdrawal (especially a good quality Indica brand) but the inhibition of GABA can possibly lead to a seizure.
If Dave was taking bars of xanax, it takes years to get off and taper down to where the effects of the xanax are gone and if pot helps him, then he is stuck with that. Xanax withdrawal is insane if you have ever been through it.
But good story; thanks for sharing.