I would like to slightly disagree and say yes absolutely the contractor can communicate with the client. It is critical to remember that the contracting parties are the Client and the Contractor, the PM for all his powers is a creature of the project given specific power and obligations by the Contract but outside those specific powers and obligations the PM is of no relevance to commercial discussions.
This is not a multiparty contract (such as the PPC2000). I do agree however that the right forum is a risk reduction meeting following EWN.