Related for Shabbat 306:6

Five may not give terumah, and if they do so, their terumah is not considered terumah:A heresh (deaf-mute); an imbecile, a minor, And the one who gives terumah from that which is not his own. If a non-Jew gave terumah from that which belongs to an Israelite, even if it was with his permission, his terumah is not terumah.

A Gentile that separates the terumah of his friends (חבריו not חברים per the GR"A), or of a Jew, even with permission, his terumah is not [valid] terumah (Ter. 1:1). It so happened in Pinah (alt. "in Pigah" =בפיגא), that a certain Jew said to a Gentile, "Separate the terumah of my threshing floor." And he separated the terumah, and [subseqently] the terumah fell back onto the threshing floor [becoming mixed with the rest of the produce]. And this matter came before Rabban Gamaliel (alt., "Shimon ben Gamaliel") and he said, "Since the Gentile separated the terumah, it is not [valid] terumah," [and thus the resulting mixture is not forbidden]. Rabbi Yitzhak says, a Gentile that separates the terumah of a Jew, and the owners [of the produce] remain near him [throughout the process], his terumah is [valid] terumah.