[US] Help me convince my FIL to stop texting his « scammer friend »

Last night, in the midst of a family dinner, my father-in-law got a text and said « oh, it’s just my scammer friend, I’ll get back to her later ». When pressed, it turns out this person replied to a public Facebook message of his, and now he’s been texting back and forth with them for months now. A cursory review of the FB account in question makes it painfully clear that this is a dummy account, and that this is a scam artist. My FIL seems to understand this on some level, but insists « if I’m careful about what I tell her, what’s the harm? ». Meanwhile, he’s sent this person literally *thousands* of messages over multiple months. What arguments/examples/anecdotes can I give him to convince him to stop texting this scammer?