Hi Claire,
None of your Disney Dining Plan meal entitlements will be deducted until you have actually arrived at the Walt Disney World Resort and started to enjoy your meals.
When you are seated at each of your Table-Service Advance Dining Reservations, your server will ask you if you have the Disney Dining Plan. If your Mum would like to pay cash for her meal, just let your server know. They will be happy to arrange two separate bills for you so that when you come to pay, your Mum can pay cash for her meal and your Disney Dining Plan meal entitlements will be deducted to cover the bill for the rest of your travel party (aside from gratuities and any other items which are not covered by the Disney Dining Plan).
It's really easy at any of the Quick-Service dining locations as your Mum can just pay cash for her meal (if she would like to do so), whilst the rest of your meals will be deducted from your Disney Dining meal entitlements.
If you have the time, I would recommend that you take a look at the
Dining Plans - Frequently Asked Questions section of the Walt Disney World website as this contains lots of other useful information about the Disney Dining Plan.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma