1. The lore hints it. When you search all the books throughout all 4 acts, you repeatedly get the message "Andariel helped Diablo from time to time."
2. A-d-r-i-a. All 5 letters are in the word Andariel. Come on, of all the possible names Blizzard can use for Leah's mother, why would they pick these 5 letters so specifically?