Hi Steffen,
The ultimate responsibility here lies at the manufacturing location. During production, they are responsibile to employ manufacturing practices that will eliminate, or at least reduce to an acceptable level, foreing material complaints of this type. Coming from the canned food industry, one of the first things that I would do is ask for the sample. Occasionally, there are characteristic changes that occur during thermal processing which result in coagulated, blob-like items that can often be mistaken as a worm or a bug. While you need to be sensitive to the consumer during this complaint, you also need to be completely sure. I recall having a small rodent returned one time and the consumer was very adamant that it was in the can when they opened it. We sent the rodent to an outside lab to be tested, and they verified that it was never thermally processed, and therefore could not have been in the can. There are all sorts of possiblilities - but without proper investigation you are left only with assumptions. To touch on Charles. C's point, if it is indeed determined that it is a worm, further (root cause) investigatoin should happen at the manufacturing facility accompanied by corrective action that will mitigate recurrence.
Hope this helps,
Chris