Can a "kosher eating only" Jew who receives a coupon for a free meal at a non-kosher restraurant give that coupon to another jew, even if they don't keep kosher? I know you are not making them go and eat non kosher food, or even know for sure that they will use it, but you are making it easier for them and even encouraging them to eat non kosher food.
It's prohibited. While not the Biblical "lifnei iver" as he could eat non-kosher with or without you, it's the rabbinic "mesayea y'dei ovrei aveira" as you're still helping facilitate his transgression here.
There are various loopholes applied to get out of the prohibition of mesyaea if there's a good need, e.g. many rabbis allow inviting non-observant people for Shabbos even if you know they'll drive. But in a case of a non-kosher-restaurant coupon, I just don't see it.
You could certainly give it to your non-Jewish next-door-neighbor or co-worker. There are enough times that they have done (or will do) a favor for you, that it's only fair. ולפיכך אין בזה משום ״לא תחנם״
(Yes if you gave it to a non-Jewish acquaintance, you would be deriving benefit from the coupon in the form of an "I-owe-you-one", but that's okay for virtually all non-kosher foods other than Ruminantia-meat-cooked-with-Ruminantia-milk, and maybe non-kosher wine according to Sephardim.)