Well, you could say she cares or feels sufficiently stirred up enough by the video to take the step of actually commenting on it, albeit rather defensively by labelling it fake.
There must be unknown others who would watch the video, possibly also defensively like to think it's fake but not even have the courage or level of caring to bother to comment.
She has commented. Good on her. If I replied, I'd acknowledge she cares enough about the topic to comment on it (praise is always a good way to soften 'opponents') and then perhaps I'd 'wonder' to her what her reasons are for concluding it's a fake?
Does she know something you don't? Can she produce evidence it's a fake? Does the video contain factual errors that she could point out to you?
Innocently draw her out rather than counter her comment or make judgements about her evident defensiveness. She may or may not respond - but whatever she does, she cannot say she's been slammed for disagreeing with you - which she would be if she ever disagreed with the Watchtower.