The manufacturer of the nacelle is not necessarily the certificate holder under part 25. Since it only fits and works on an Airbus I would look to Airbus as the responsible party for its operation in-service.
Airbus is most certainly responsible for the Part 25 certification - with the notable exception of the actual engines (that are certified separately) the airframer is responsible for certification. But apparently Rohr/Goodrich was responsible for the design of the cowl and the associated latching system and presumably would be responsible for any sort of redesign to avoid future cowl separations (although Airbus would likely be 'involved').
However it's an Airbus aircraft, they own the TCDS, and ultimately Airbus has the responsibility.