The FA should have initiated contact in a more professional manner within company policy. Using his company provided webmail account he can send the email "Recently I had the pleasure of serving you on Flt XXX. I hope you found the flight enjoyable and look forward to serving you again, etc."

It's a simple "marketing" type email and if the pax is interested, then voila he will respond accordingly. If not, "no harm no foul."

While I would not report the FA to corporate (just bad karma), the OP'ers recommendation on sending a short note to that FA looks to offer the best outcome for all involved.

Last thing you want is for that FA to get fired, end up at another airline and having him serve you in F on a LAX-CDG flight... "I hope you enjoy your salmon, I seasoned it especially for you!"