I agree with Kitty, but really the way this question is posed is way too vague to really tell exactly what happened. Obviously, you don't want to name names, but it would help to be specific on how you're related to the various people.
There are some basic things though, that seem to be missing. Having done the AncestryDNA test, myself, it's obvious to me that if this mysterious person has a DNA match to a relative, then that relative would also have a match to him showing up on her test. So, assuming your known relative is a reasonably trustworthy person, she could tell you if this guy is a for-real DNA match without you having to believe anything he says.
Further, AncestryDNA will give you a sort of estimated relationship, but unless you're a full sibling, there are generally several possible relationships that would match the centimorgan count.
Our investigator doesn't sound like she's up on this stuff, and that surprises me, given the successes she claims.
I would also add that if mystery person is a fairly close relative (somewhere around a second cousin or closer), they would almost certainly show up on your test, too if you've also done AncesrtyDNA.
I don't know what this talk of test numbers is about. That's only relevant for GEDmatch, as far as I know (and it's more properly called a "Kit Number"). If everybody's out on GEDmatch, then this stuff gets even easier.
I would think all that stuff would be the foundation for any of this sort of discussion, so I really can't tell what she's talking about, and it's concerning what other "information" she could be taking about being shared.
