The British actor was reluctant to believe his girlfriend when she said she had only kissed the Snow White and the Huntsman director - who himself is married - several times, but after she proved it to him by giving him passwords to her email and voicemail accounts, he accepted her explanation.
A source told Hollywoodlife.com: "Kristen and Rupert both told their respective partners that they didn't actually have sex.
"Kristen was absolutely steadfast that she only engaged in several make-out sessions with Rupert but never slept with him. Rob didn't believe her in the beginning but as time wore on he accepted her explanation. Kristen has given Rob passwords to her voicemail and e-mail account to prove she has nothing to hide and will do anything to get his trust back."
The pair - who met on the set of The Twilight Saga movies - are now believed to have reconciled.
A source said earlier this week: "They are living together and have reconciled."
Rupert Sanders and his wife Liberty Ross may have also made up, as the British beauty was recently seen with her wedding ring back on.