Memory Theft allows you to put any face-up exiled card with an Adventure that player owns into their graveyard, not just one that was cast as an Adventure and exiled.

An effect may refer to a card, spell, or permanent that "has an Adventure." This refers to a card, spell, or permanent that has an adventurer card's set of alternative characteristics, even if they're not being used and even if that card was never cast as an Adventure.

If an effect refers to a card, spell, or permanent that has an Adventure, it won't find an instant or sorcery spell on the stack that's been cast as an Adventure.

You choose which exiled adventurer card, if any, to put into that player's graveyard after seeing their hand. You may do this even if that player has no nonland cards in hand, or even no cards at all.