Can I give a בשר בחלב restaurant coupon to a goy that would allow him to get a free בשר בחלב meal? Would this be considered getting הנאה from בשר בחלב? (Assume this coupon can only be redeemed for a בשר בחלב meal and not just drinks or other meals that aren't בשר בחלב.)
The באר היטב in סימן צד - דין התוחב כף חולבת בקדרה של בשר 94:3 says:
(ח: בהנאה. (ר''ל הקערה צריך שבירה והמאכל זורקו לבית הכסא אבל לא לפני הכלב אפי' אין הכלב שלו
The Shulchan Aruch says "If a pot has milk & meat cooked in it, everything is forbidden, including the pot."
The Be'er Heitev adds that in the case of an earthen vessel that cannot be kashered, the vessel needs to be broken and the food thrown down the toilet, but not given to a dog, even if you are not responsible for feeding this dog.
It seems that the mere fact that you feel good about having fed a stray dog is considered "benefit".
it would follow, I assume, that the fact that you know that another human got a meal, would surely be forbidden.
OTOH: In the case in the Shulchan Aruch you're feeding the dog the forbidden mixture. In your question, you're simply enabling the possibility. There's also the possibility that the goy will pass on the voucher to another goy - totally removing you from the prohibition/benefit. So maybe the cases aren't similar.
In סימן צב - דיני אם נפל חלב לקדרה של בשר the Shulchan Aruch says that if meat fell into boiling milk, one gets a goy to taste the milk, and if he can't taste any meat then the milk is kosher.
Note: we don't seem to rely on this method, but that's not what we're discussing. See your LOR for practical applications.
א כַּזַּיִת בָּשָׂר שֶׁנָּפַל לְתוֹךְ יוֹרָה שֶׁל חָלָב רוֹתֵחַ, טוֹעֵם הָעוֹבֵד כּוֹכָבִים הַקְּדֵרָה, אִם אָמַר שֶׁיֵשׁ בָּהּ טַעַם בָּשָׂר, אֲסוּרָה, וְאִם לָאו, מֻתֶּרֶת, אֲפִלּוּ בְּפָחוֹת מִשִּׁשִּׁים, וְאוֹתָהּ חֲתִיכָה אֲסוּרָה.
Point being that one may even feed a goy one's own meat & milk mixture for one's own benefit. Surely one can give him a voucher which would enable him to do same.