At first glance, it seems that your friend is making Kiddush at a time when he is not obligated to do so on behalf of those who have presumably accepted (or are now accepting) Shabbat and are thus obligated in Kiddush. In general, one who performs a mitzva on behalf of someone else must be obligated in it to the same degree as the one for whom he is doing it.[1] On the other hand, this requirement may be somewhat theoretical, as one who has already discharged his obligation in a certain mitzva can perform it, even with its beracha, in order to enable the fulfillment of the obligation of one who has not yet done so (based on the concept of arvut[2]).[3] We must thus consider the following: Is your friend considered obligated in Kiddush because it generally applies to him, just as in the case of one who already fulfilled his obligation? Or do we say that if the time is such that the mitzva does not apply to him, he is not considered obligated at all?
In order to resolve this dilemma, we need to consider cases conceptually similar to this one. Acharonim cite a Yerushalmi[4] that says that residents of un-walled cities may not read the Megilla on behalf of those from walled cities (whose obligation is a day later, on “Shushan Purim”). We see from here that one who is generally obligated in a mitzva but not at the present time is not considered obligated with regard to performing the mitzva for one who is obligated at the time. However, Rabbi Akiva Eiger[5] raises a distinction to suggest that our case is more lenient than the one discussed in the Yerushalmi. Here, one who has not accepted Shabbat can accept it and immediately become obligated in Kiddush. Therefore, the obligation is considered relevant for him even before he actually accepts Shabbat, and perhaps he may therefore recite Kiddush on behalf of one who accepted Shabbat. However, Rabbi Akiva Eiger leaves the matter as an unresolved doubt.
Several contemporary poskim tried to resolve Rabbi Akiva Eiger’s doubt. Rav S.Z. Auerbach[6] cites Tosafot,[7] who says that one who did not eat may recite Birkat HaMazon on behalf of one who did eat. Rav Auerbach reasons that the one who did not eat is considered obligated in Birkat HaMazon because he could eat and thereby become obligated, in line with Rabbi Akiva Eiger’s logic for leniency. However, Rav Auerbach makes a distinction between Birkat HaMazon recited by one who has not eaten and Kiddush recited by one who has not accepted Shabbat. The mitzva of Birkat HaMazon theoretically applies to all people at all times, as long as the circumstances are that they ate. In contrast, Kiddush on Friday afternoon is not considered a relevant Kiddush for one who has not yet accepted Shabbat, even though he could accept it if he wanted to.
On the other hand, we should consider whether it is clear that Kiddush is irrelevant before Shabbat. The Rambam apparently does not accept the concept of tosefet Shabbat (the ability/obligation to usher in Shabbat early),[8] yet he says that it is possible to recite Kiddush toward the end of Friday afternoon.[9] This lends credence to the concept that Kiddush (as well as its parallel, Havdala) applies and may be recited soon before the time of the transition between weekday and Shabbat. Thus, the obligation of Kiddush may already apply on a certain level soon before Shabbat even for one who has not yet accepted Shabbat, making him connected enough to Kiddush to enable him to recite it for those who have accepted Shabbat.[10] While it is difficult to rely on this thesis alone, which is certainly not unanimously held, it can be considered seriously when contemplating grounds for leniency in various cases.[11]
Rav Auerbach was hesitant to allow Kiddush in such a manner on a regular basis. However, Rav Ovadia Yosef[12] and the Tzitz Eliezer[13] were willing to rely on the more lenient position of Rabbi Akiva Eiger and allow one who has not accepted Shabbat to make Kiddush for others. In a case such as the one you describe, where one is doing an extra mitzva of chesed with this Kiddush and it would likely be difficult to find a replacement who can make Kiddush without qualms, we feel that it is proper to be lenient on the matter.
It is important to note that while it is proper that someone who is hearing the Kiddush should be eating subsequently, this may not be an absolute necessity.[14] (Details of this issue are beyond our present scope.)