The intuitive answer to this question is clearly no. However, the 10th senate of the German Patent Court had to decide a fairly weird dispute among two manufacturers of tins for fish. One of them found that the description of the drawbacks of his tins in the specification of the granted patent was clearly wrong and detrimental for his reputation.
The plaintiff had first sued the patentee on the basis of an unfair competition claim through all instances up to the federal supreme court, the 1st senate of which (responsible for unfair competition) decided that the exclusive jurisdiction for the contents of a patent specification was at the Patent Office and the Patent Court. So the buck was passed to someone else..
Of course, derogative remarks in the specifications do not count among the limited number grounds for oppositions or nullity in the German procedure (which is comparable to Art. 100 EPC in this regard), it is not possible for a competitor to enforce his right to have the unfair remarks deleted in one of these invalidation procedures (unless the incorrectness of these remarks results in a lack of inventiveness, which may be the case when the alleged invention arguably resolves the non-existing drawbacks).
The German Patent Court has now published a notice (10 W (pat) 21/06) that an appeal filed by a third party against a decision to grant may be admissible under exceptional circumstances, i.e. if the passages under dispute are unrelated to the invention, obviously wrong and abusvive taunt. However, the passages did not qualify as obviously wong in the case under dipute.
So the suprising answer is: yes-a third party is entitled to appeal a decision to grant - under exceptional circumstances.....