Should the adjective agree with the subject or the object?

The adjective always agrees with the subject, never with the direct object. For example "she is gone" and not "she are gone", even if you're talking about multiple people - the subject is "she".

Don't forget that the confusion often comes from state verbs with a direct object - like "je les ai trouvées intelligentes" (I found them intelligent). There the adjective "intelligentes" agrees with "les" (the direct object), not with "je". The thing that really helps: mentally replace it with an active verb. "Je les ai trouvées" → "j'ai trouvé elles" = agreement with them. Versus "elles sont parties" → subject "elles" = agreement with them. Once you see that difference, it becomes way less tricky.

You're mixing up different cases, both of you. The guys are right, but the second one is missing a distinction: with a linking verb (be, seem, appear), the predicate adjective always agrees with the subject, even if there's a direct object nearby. But with verbs like "find, make, leave" + direct object + adjective, then yes the adjective follows the direct object ("I found them intelligent"). It's not just a subject vs. direct object thing - it's the nature of the verb that changes the rule in the first place.