Presumably she already has a BT email account that she can access on a computer.
If not you will need to set one up for her.
See link how to do that.
Once you have the email set up and working on a computer you can then set it up on her iPhone.
You are best to set it up manually using the following settings.
When you say "when I try to add it into her phone it just can not find it" what exactly is happening?
You are best to turn off any other device that is used to access the email account while doing the set up.
Try following this guide and if there are problems take a note at what stage it happens and what exactly is happening.
It is best to go through settings >mail contacts, calendars > add account… > don’t choose the ‘yahoo’ or any of the named options, tap the ‘other’ option from the bottom of the list > Add Mail Account and enter the following details:
On the next screen it will want you to input incoming and outgoing mail server details:
Incoming mail server settings:
Outgoing mail server settings:
Additional setting changes