Well....in real life, the chance of you not knowing it is remote. You would always follow the gender of the person or the noun.
If you talk about windows, for instance, it's "altra". If you talk about "icecreams", it's "altro".
Even for unknown things you would end up choosing a gender like "altra (cosa)", "altro (oggetto)".
In this person case you might even use "altra (persona)".