One of the great talents of the yetzer hora that comes forth after one has done teshuva, is the ability to send the person back in their mind to where they were, which brings on tons of guilt. The yetzer hora reels with frustration when a person does real teshuva, and his tactics are many. What you are describing is a very accurate method of the Y”H doing his job well.
An eitza that is given to those who are trying to FEEL that they have done real teshuva, is to work on believing that Hashem has accepted it. If one has changed their behavior, has committed himself never to do that again, then it’s done. Gone! Hashem has no interest in people wallowing in guilt their whole lives, or even to the extent that they can’t get past what they did.
Teshuva is one of the greatest gifts Hashem has given us. I would suggest learning Sha’arei Teshuva, which should boost this person’s realization that they are truly loved by Hashem, and they have the ability to conquer the Yetzer hora’s ploys to make them feel negative.
Hatzlocha!