The claimant needed to sign a NDA
And there it is. The ever-expected excuse.
It is pretty easy to come up with a couple dozen reasons as to why you can't provide some information: you want to imply, "the onus is on the accuser," but of course when we look at the bigger picture, what we see is that someone is claiming you helped them... without any evidence.
This means that pragmatically, we know not whether the user in question is you, me, or anyone that actually was helped by you.
You might say that this is similar to the scam accusation, but the issue is that by ignoring a
scam accusation rather than
positive feedback you are merely showing a selective bias against any dissent. This leads me to assume that despite any scam accusations that have come out, are coming out, or will come out, you will refuse to answer them regardless of validity.
That, in my opinion, is not behavior indicative of a professional service.