Pronoun for her glasses

The pronoun for 'her glasses' is they as a subject and them as the object. Example:

• Jane can't find her glasses. She thinks she left them in the library. They have a red frame.

However, if the full term is used, 'her pair of glasses', the singular pronoun it can be used, although it rarely is. Example:

• Jane may have left her pair of glasses in the library. She said it has a red frame.