sam2 – I don’t see how “eid echod” applies here, and indeed in most such questions these days.
Eid Echod tells you that the person conformed with halachah.
It is clear that different people have different standards in kashrus, and have different opinions about individual hechsheirim. Even if my friend is trying to do the right thing, there is a very real possibility that he uses products/hechsheirim that I don’t use.