Tosfos (Yevamos 83b, DH Ein) asks a similar question to yours. He asks that if we say that a person cannot prohibit what is not his, then why, if someone puts a Neveilah in his friend's pot of food, or if he puts milk in his friend's meaty dish, does this become forbidden? Tosfos answers that the rule of "Ein Adam Oser Davar she'Einu Shelo" applies only to something which depends on thought, while if Tereifah food was cooked with kosher food, or if meat was mixed with milk, this does not depend on the thinking of the person who did the mixing, but on the reality that the forbidden mixed with the permitted, and it makes everything prohibited.

According to this, your question is answered, since it was an action that was done to the woman, and this prohibits her to a Kohen.

Kol Tuv,

Dovid Bloom