Ok, if by lustful thoughts he means he is fantasizing, then quite possibly.
However, if he is just having a thought and not indulging in it, then he is not culpable for that. This is why it is important to understand there is a difference between thoughts and ourselves. If he has intrusive thoughts, this is especially necessary.
We are not culpable for mere thoughts that pop into our head.
Now, the “grey area” will be how one defines indulging. In this case, it is proper to define it as actively, purposefully thinking about it.
I know how painful this can be. One time, it was the end of lent, and it was Easter I believe. I had not eaten anything for two days, I confessed, and the he thought of receiving the Eucharist at this traditional Latin Mass I was at after my fast and confession made me cry. I was so elated for it. But, then a thought came up. A gross image. It ruined the whole thing for my subjective experience. I was wondering if I had sinned, if…and then received and was so strung up with doubt and sadness. I’ve had intrusive thoughts that would be terribly heretical or sinful, and what I would (and still) do was to think of a way to finish the sentences that were acceptable. It would happen constantly, I couldn’t focus, and I would be mentally exhausted.
One may never be completely healed, but it can get much better. It takes a lot of prayer, a lot of understanding, and a new way of looking at things from the proper angle.
I once read a comment on Quora, and I can’t tell you how relieved this just made me feel, that said, some people get obsessed over a thought. But it is just a thought.
And that is so true.