I chatted them and got this:

ME:We are a DME and received a referral for a patient who has Medicare Primary and TRIWEST secondary. Medicare will pay 80% of the allowable - will Triwest pick up the 20% even though we are not contracted

THEM:Triwest will only only process as primary with a referral the provider does have to be contracted

ME:ok thank you, so for our purposes, the patient will be responsible for the 20%

THEM:If you received a referral from VA contact them for further assistance 877-881-7618