Let me see how I'd use "deren" as a native speaker:
1) as relative pronoun referring to either one female person or a plural:
Die Frau, deren Sohn im Auto saß. = The woman whose son sat in the car.
Die Eltern, deren Sohn Biologie studiert. = The parents whose son is studying biology.
2) as demonstrative pronoun referring to either one female person or a plural:
Wir trafen Marianne und sprachen mit deren Sohn. = We met Mary-Ann and spoke with the latter's son. (*)
Wir trafen die Müllers und sprachen mit deren Sohn. = Wir met the Millers and spoke with their son.
(*): In most contexts, this sounds pretty unnatural to a native speaker's ear. One would definitely prefer the use of a possessive pronoun:
Wir trafen Marianne und sprachen mit ihrem Sohn. = We met Mary-Ann and spoke with her son.
So, yes, you are right, getting back to your question, it could be "her son", but then one would usually say "ihr Sohn", not "deren Sohn".