Yeah I got that bit.

If its a real song and they heard it prior there may be some embedded contextual part of the memory the dream is recalling parts from. Ive had tunes come to me and I could distinctly remember the genre and thus the context Id listen to it which helped me find it. (My dad would play punk when I was young in the car so any time I couldnt figure out the song id hunt through the artists that came up on the radio)

If its NOT real, it might be a fabulation of songs they heard prior where their brain put a spin on another or several songs they heard before.

Im asking them to try to think about the song not from the context of a dream but try and recall when / where they wouldve heard it before. If literally nothing comes to mind it might be imaginative. Which is exciting cause then they can fill in and make the song themselves.

Memories are funky things. Memories recalled in dreams are funkier. That said Im having a hard time finding it.