At a chuppah in which, after the chatan says harei at, the kallah responds with harei ani mekabelet etc. when should the kallah say it? Before the ring is given? After he places ring on her finger?
I would have her say it before accepting the ring, as it communicates her da’at before undergoing the act. However, some have concerns around נתן הוא ואמרה היא when she says it before to frame the kiddushin (NB: this sugya in the Gemara (kiddushin 5b)and Rishonim is when she says הרי אני מקודשת לך or something similar. Meaning – it accurately describes the kiddushin, she is just the one doing the amirah. It is not relevant to contemporary discussions of her saying הרי אתה מקדוש לי or the like, which completely mischaracterizes the kiddushin).
So, as far as אמרה היא goes – almost all Rishonim say it does not apply when what she is saying is unneeded and superfluous – such as all our cases, where there is עסוקים באותו הענין and the chatan already did his amirah. Nevertheless, some have lingering concerns based on the Ran, that since it is a kinyan and she is the object being “purchased,” then only the man can be active. She is not supplying active, participatory da’at, only “stepping out of the way” and not objecting and allowing herself to be purchased. According to this, she has to be fully passive, possibly even when he active participation is unnecessary.
So, based on this concern, I would have her say הרי אני מקבלת טבעת זו להתקדש לך – i.e., I am accepting it for the purpose of the kiddushin, but I am not doing a declaratory act vis-a-vis the kiddushin itself.
A recommendation I would offer is that instead of, or in addition to doing this, one could have the kallah say – before the chatan speaks – הריני מוכנה לקבל ממך טבת קידושין ולהיות מקודשת לך כדת משה וישראל. Since it is future oriented, and says “for the purpose of becoming” – there is no declaratory, מעשה אמירה, act AND it has her SPEAKING FIRST – being the first voice that the kahal hears, which can be very powerful.
You may wish to see this brief analysis by Rabbi Saul Berman shlita, from July 2000. Rabbi Berman does not address the position of the Ran in this discussion: https://www.aishdas.org/avodah/vol05/v05n081.shtml#18