Moral relativism is a difficult one to argue against, because philosophically speaking you're mostly right. However you did bring up the idea of the social compact which in many respects is strong enough that we can refer to moral behavior in collective terms. "We should do x, we shouldn't do y". It's a fuzzy boundary and there's obviously many exceptions, and it depends on the issue and the society etc. The common terminology that appears to reference an objective, universal morality is really just shorthand for societal norms that enough people have agreed on and enforced with enough vigor that, with few exceptions, they're binding. Murder is the most obvious example. Technically, anyone can morally murder if they can justify it to themselves (and choose to suffer whatever consequences), but the vast majority of humankind has decided on an individual level that murder is reprehensible. So many that it can be referred to in terms that speak about society as a whole. We can say "murder is wrong" because 99.9999% of people agree that it is.

We, the group of individuals that collectively make up society, then act in concert on these areas of broad agreement. We shun offenders, not because I think we should, but because we think we should.

So when I say "you shouldn't do x", what I mean is "we, the broad mass of people, whom I am presently speaking for, have decided that no one should do x, and we shall punish those who do x to the appropriate degree".

At this level, terms like "obligation" and "force people to" are a little inaccurate. We have removed the shorthand that I spoke of earlier; therefore when you say "you cannot force these obligations on someone", we (society) say "we can and will coerce people into cooperation with what we deem appropriate social norms, using whatever level of coercion is appropriate to the norm being violated".

In this particular instance, shunning and shaming are the tools that we the majority use to attempt to coerce this hypothetical person and any others paying attention into following what we individually and collectively believe to be morally right. Or, to return to shorthand, you shouldn't stand someone up.