Teri Copley, the ex-gf John's accused of an alleged affair, denies cheating on him ... she says they were already broken up when Stamos found her in bed with Danza.

The model/actress confirms part of John's story ... that he came over to her place to find her naked in bed next to a passed-out Tony ... but she told People she and John had already ended their relationship.

Teri told the outlet, "I wondered, 'What was John doing there?' because we had broken up. He just looked at me and shook his head, and walked away."

As we reported ... John reveals the alleged affair in his upcoming memoir, "If You Would Have Told Me," claiming he also peeked into Tony's Porsche, which was parked at Teri's place, and saw an autographed poster of her half-naked with a note that read ... "My Dear Tony, I'll love you forever. XO, Teri."

John said he was madly in love with Teri at the time ... but it sounds like she didn't feel the same way.