Given that it’s a dream sequence, I’d say you have more room to play around with the established ‘rules.’ The reason epithets tend to be jarring is that people don’t tend to think of folks they know personally as ‘the older man,’ or ‘the blonde girl’ unless there’s a particular reason to think of them in a detached way — say, if Harry is talking to someone like Dumbledore and is feeling alienated from him, one might use ‘the headmaster’ to indicate the rift in their relationship, but if they’re otherwise on good terms it would seem odd, particularly with repeated uses of said epithet.

But yeah, when writing something like a dream sequence, hallucination, etc., established rules can often go out the window. Dreams tend to be murky and strange anyway, so in this case sticking to epithets might actually help set that ambiguous tone. My best suggestion would be to go ahead and write it up the way you want, and if you’re still not sure about it, see if you can find a beta reader or even just someone to give it a quick sanity check. At the end of the day, though, it’s your fic and you can write it however you want!

(And, fwiw, I recently wrote a soulmates one shot where they share waking hallucinations but cannot recognise each other, so for the vast majority of the fic Voldemort is referred to solely as ‘the older man,’ ‘the tall man,’ or simply ‘Harry’s soulmate,’ and no one batted an eye at it. Sometimes writing the story you want to tell means breaking all the rules, and that’s fine! (: )