but if we don't? i mean if we don't surely know whether or not the thing we talk about is feminine
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)".
I agree with many of the comments already posted - i.e. out of context surely you would need to indicate that the other one is actually feminine.
Altra -female so my translation as who is the other woman is accurate....DL is useful but still wildly unreliable
Wouldn't think so, as "l'altra" has a definite article, while "who else" has a clearly indefinite ring to it ("who is that other person I see or have heard of" versus "who might be others involved in this"). The direct translation would be "chi altra/o".