Gun Advocacy Investigation

I am working on a investigation of a gun advocacy group to do so I must work with an outside department. While on duty in an adjacent county I have been assigned to work with a officer from their department. During the course of our investigation we take a short meal break, I become aware that the officer I am working with is being given free food from a local convenience store. I have been offered to do the same, being told “if your hungry go inside and get what you want”.
I am working on an investigation of gun control and not police officer conduct, I would advise the officer that taking of gratuities is not allowed and I won't be a part. I would also note it in my file as part of the days activities reporting it to my chief upon return
to the department. I would not participate or enter the store in question.
I am concerned the officer just probably wants to save a buck, but this could lead to more advanced forms of bribery. Being that we a looking into a gun advocacy group I would concentrate on that issue, and do a complete and in depth job concentrating on the assignment. I would bring it up to my superiors upon returning to my department and let the department take what ever action they feel appropriate.