Dear Gerrard,

Thanks for the question.  

God is omniscient in His Nature, but His creatures can partake in the divine nature by grace.  

God shares his knowledge because we partake in His life, and that is true of Mary, or any Saint that abides in God.  So, if someone was asking Mary for her prayers, God, who hears and sees all, can share that directly with her.  So Mary is not omniscient by nature, but by grace she can be privy to what God alone knows.

Peace,

Bob Kirby