Pork substitute for Game Meat Pie
Duck fat.

You could use butter, of course, which would make it delicious like everything made with butter is.  But duck fat has a higher melting point than butter and in that respect is more like lard.  Duck fat is super delicious.  Also it seems to me duck fat is more in accord with the overall ethos of a game meat pie.

You could buy plain duck fat.  Or you could first have your friend over for roast duck, and you would then save the rendered fat and have her over for the game meat pie later.  Do the duck in a way such that any flavor in the fat will match what you intend with the game meat pie - for example duck with Moroccan spices would be great but those spices might then crash the game meat pie party.