Your plan is spot on. Bill might not be a memorable chap, and if so a reminder of your work together would be helpful. No harm in asking. If you didn't really help Bill all that much, then let him know that your feedback on his couple uses of the writing center won't offset his cheating/failing academic progress in other subjects. It's really not the same situation if Bill has flunked out of chemistry too many times while Adam has flunked out of his literature classes too many times. In one situation your feedback may be central, while in the other it's busywork. One way or the other you can't help out the student who knows you helped out his friend without some kind of refresher...gonna have to get a quick aide-memoire. It could also be a diplomatic yet direct response to pose the question to Bill how your feedback could at all make a difference in the first place. Make him tell you.