If she has her own email address, you would create a new "Account" in Windows Mail using her Username, her Password and the POP3 server address of her provider. For outgoing mail, she must still use the SMTP server of the ISP with which she's connected to the internet, which will be your Orange SMTP server.
If you're trying to set up a second email address for her with *YOUR* Orange ISP, you need to establish if you can have multiple addresses and, if so, whether it is possible to retrieve mail separately for each address. The way this is generally done is by using the individual email address as the mail login. For example, if your address is username(at)orange.net you would create new addresses for each of you, such as:
Your ISP Username remains the same, but moves to behind the @ so that you can have your chosen name before the @
To retrieve your separate mail, you would have separate "Accounts" in Windows Mail and log in with the full individual email address. This is the conventional method, but I cannot confirm if this is how Orange works.