Shalom,

If there were not two kosher witnesses watching the giving of the food to the girl, then the question does not get off the ground. Besides that, Mishloach Manos is usually done by way of a third party between the giver and the recipient. Granted there is Kiddushin through Shlichus, but only if it is abundantly clear that the third party is, in fact, acting as a Shaliach Kiddushin for the Meshalei'ach.

In Rav Moshe Feinstein's famous monumental Teshuvah on the subject (E"H, I, 82), a group of youth were playing "marriage" and there were two young men acting as witnesses and the "Choson" said Harei At Mekudeshes etc. to the "Kallah", none of which presumably exists in our case, and still Reb Moshe ruled that she did not require a Get because it was clear that they were just playing a game (it is interesting to note that the Teshuvah is exactly 100 years old this year and that the story behind the Teshuva took place in Russia, not the more free wheeling America).

The Rem"a in Hilchos Megilah (695:4) says that a man should not send Mishloach Manos to a single woman, not because we are worried that the accepting of the food will constitute a Ma'aseh Kiddushin, but rather because the food might be construed as being Sivlonos - presents that are sent between a Chasan and a Kallah after the Kiddushin - and therefore there would be a suspicion that there had been a proper Kiddushin beforehand.

Kol Tuv,

Yonasan Sigler

This is not a Psak Halachah