According to Einsteinian (what I understand to be Minkowski-an) special relativity theory there are not relations of absolute simultaneity. In other words, if A is simultaneous with B, and B is simultaneous with C, it does not follow that A is simultaneous with C. This means at least that simultaneity is not transitive.
Now, supposing that we maintain that A’s being simultaneous with B, and B’s being simultaneous with C, does rationally entail that A is simultaneous with C (that we rationally intuit as a necessary truth that simultaneity is transitive), what are we to respond to the physicist?
First, it should be noted that according to Lorentzian relativity, which presents itself as an alternative to the standard Einsteinian view, relations of absolute simultaneity can and do exist. Therefore, we could just suggest that the physicist who prefers the standard Einsteinian view to the empirically equivalent Lorentzian view, is mistaken and ought to convert. However, Lorentzian relativity theory seems to vindicate the A-theory of time, and we may not want to go down that road (I’m not sure I do).
The only other thing I can think to say, however, is that if I rationally intuit the transitivity of simultaneity in such a way as would make it seem self-evidently true to me, then I have a defeater for taking the Einsteinian view seriously. However, I may not need to run all the way to scientific skepticism. I may just want to say that whatever the Einsteinian means by saying that, for the purposes of her preferred physical model, there are not, in that model, relations of absolute simultaneity, simply doesn’t translate to metaphysical reality, or at least doesn’t translate to what I have in mind when I reflect on relations of absolute simultaneity.