I was approached by a user in regards to a possible Ponzi ICO.
Emphasis mine. This is what concerns me here.
If the campaign in question is simply stating facts, then it's not really any different to negative campaigns which takes place in many political systems around the world. Some people do not ethically agree with negative campaigning, others think it is fine, but (in most jurisdictions) it is certainly not illegal.
If, however, the campaign is extrapolating or implying things which may or may not be true, which it sounds like it will be since you have said
possible Ponzi and not
definite or proven Ponzi, then it becomes more of a smear campaign and potentially libelous, which then could become illegal.
As others have mentioned, the motive behind this is concerning as well. A third party that is doing this out of pure altruism at their own expense simply to warn others is far different than a direct competitor attempting to increase their own business. Given that there are literally hundreds of Ponzi ICOs, what is so special about this one?