She seems talking about one person who is "particular person", but why does she use "them" which is plural to refer a person?

Rebecca

The pronoun "them" is used here because the gender of the person is unknown. You can use the pronouns "they" or "them" in the singular form whenever the person they refer to is unidentified and the gender is unknown, because "they" and "them" are genderless pronouns. Ex: Her friend seems really nice but I haven't met them yet.