The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other.
I think this is the most debatable step. Certainly, in moral relativism, one does not admit a single standard. If a moral relativist said something like this, he would only mean "I think it is better, but you may not." However, I doubt any moral relativist would say something like this. More likely, it seems that a moral relativist would acknowledge the statement as vacuously true since the premise ("If you say that one set of moral ideas can be better") is false.
But the standard that measures two things is something different from either.
Considering moralities as a point in some geometric space, this is most likely, but it could also be that the standard is precisely one of the two things being compared. This minor point doesn't seem to invalidate the rest of the argument though.
The rest of the paragraph is a restatement of the two above points.
So, it seems most likely that one who disagrees with the conclusion would find it logically consistent but only vacuously true.