Depends what you mean by the distinction you're making between "distinct 'nations' of people going back deep into history" and "modern imagined communities", which I think may be a false one. Anderson himself discusses how his concept of 'imagined community' has been misinterpreted:

"With a certain ferocity Gellner makes a comparable point when he rules that 'Nationalism is not the awakening of nations to self-consciousness: it invents nations where they do not exist.'11 The drawback to this formulation, however, is that Gellner is so anxious to show that nationalism masquerades under false pretences that he assimilates 'invention' to 'fabrication' and 'falsity', rather than to 'imagining' and 'creation'. In this way he implies that 'true' communities exist which can be advantageously juxtaposed to nations. In fact, all communities larger than primordial villages of face-to-face contact (and perhaps even these) are imagined." (Anderson 2006 [1983]: 6)

So it's true modern nations are an imagined community, but so was the Scottish nation discussed in the 14th century Declaration of Arbroath, or the Danish nation described in the 12th / 13th century Gesta Danorum (both of which examples use the Latin word 'natio' / 'nacio'). Doesn't mean modern nations are any more constructed than medieval ones, in fact in many ways the medieval ones have less relation to reality, e.g. medieval European nations would often claim to have originated from the ancient Trojans or some similar heroic group. Furthermore of course in the Danish and Scottish examples, these nations existed then and exist now (though clearly have changed in the time period) so they do indeed go back into history.

As you can probably tell, I'm generally pretty critical of modernist theories of nationalism, e.g. I'm more a fan of Azar Gat than Eric Hobsbawm, but of course 'nationalist' histories (e.g. an unchanging nation reaching back to the beginning of history), that you seem to say this person believes, are are often false as well, and are the original reason modernist critiques were written.

If I was speaking to someone with a falsely overly nationalist view of history, I would emphasise how nations, not just in the modern period but throughout history, have always been in flux, and I would point out to her the points in history in which this specific nation was first in evidence, and when the changes happened. I wouldn't try and persuade her that before the modern period the concept of the nation didn't exist or was fundamentally different, as this isn't true.

Which nation are we talking about, and what are her beliefs about it? I guess some nations definitely are more recently created than others, and some nationalist myths are more absurd than others.

References

Anderson, B. (2006 [1983]). Imagined Communities: Reflections on the Origin and Spread of Nationalism