It depends on the person and the situation, which is only something you can judge. Someone who is more malicious or self-involved might deny knowing anything, because they don't want to take responsibility. An innocent person may genuinely not remember, due to whatever stress is going on in their lives.
Again, it all comes down to your intuition and the person that you're dealing with. You'll know who they are and if they are being honest or dishonest with you.