What should I do about having said something to an old friend in the past, which I suspect was incorrect religious information, but I don't remember what I said?

To be rid of Waswasa (whisperings/obsessive thoughts), we advise you to distance yourself from them, to make sincere repentance from all sins, to increase general seeking of forgiveness (Istighfar), and to recite the reported supplications, such as: "O Allah, forgive my sin, my ignorance, my excessiveness in my affair, and what You know better than I," and "O Allah, forgive all my sins, the small and the great, the first and the last, the open and the secret."

General repentance forgives all sins, even those the servant did not recall at the time of repentance, unless, had he recalled them, he would not have repented from them due to the strength of his will for them or his belief that they were good. The important thing is that repentance should encompass both the sins the servant knows and those he does not know.

If you remember the person you spoke with, be sure to communicate and offer mutual advice, encourage truth and patience, and study religious sciences. By doing so, you will have fulfilled your obligation, for whoever repents after committing a sin has done what was incumbent upon him.