If you're looking for a quick answer, on a certain low level of discourse (not yours, but your interlocutor's), I would suppose that the statement, "Self-command is a virtue," would be basically what you're looking for. Almost every major ethical philosophy I am aware of, and almost every major religion of which I am aware, makes a place for self-command in their fundamental ethics. There is the closest thing to consensus on this (of course, extreme hedonists would digress, and a definition of self-command might not be absolute, but here we look upon such people as akin to Flat Earthers and such definitions as suspended, I suppose).
The argument would not be for, "Self-command is a virtue," deductively from something else, but from the near-consensus about the assertion itself.
To keep things going, you might compare ethical to mathematical judgment regarding subjectivity, and point out that things are nowhere near as "set in stone" with respect to mathematical objectivity as might be naively proposed. This would give you a, "Well, are you going to be a subjectivist about mathematics, too, just because there isn't a strong consensus about realism vs. logicism vs. formalism vs. intuitionism vs. structuralism vs. fictionalism vs. ...?" line of response.
On higher levels, you can bring up distinctions in ethical concepts that can seem real enough, or which presuppose some kind of relevant objectivity, e.g. you might advert to the supposed difference between "the right" and "the good" and the question of a priority for these concepts. So in this case, the question, "Are either the right or the good prior to the other or are they definable independently as such?" can be seen as possibly objective, i.e. it is not a matter of mere opinion whether these concepts are ordered in one way or another. It might be in part a matter of opinion, or of stipulative definitions of specific words, but if the question can be posed strongly enough over such definitions as given, then as posed, the answer ought to have a "realism"-theoretic answer to some extent.
OTOH, ethics is heavily concerned with things like emotions and willings, which are rather subjective. Granted, then, though, that everything has objective and subjective sides, and absolute and relative ones. Indeed, there are objectively relative facts (think: "The house is to my left": it really is to my left, whether I believe it or not; but there's still a lot of relativity involved, obviously, too!).