In the fico 8 scoring model, the utilization counts for 35% of your score, where as the age of accounts counts for 15% of the score. So having a card near maxed is hurting way more than than removing that account would.
Aside from that, sounds like she has a good card on her own, plus being th AU on yours, so she is establishing a good solid fundamental score of her own. So if she doesnt need anything big anytime soon, she will be going up the ladder every few months.
Another thing to take into account is that mix of credit counts for 10% of a score, do either of you have a retail card of some sort, or vehicle loans? Remember, cards (revolving) are broken down into 2 categories, retail (such as a store specific card, or amazon store card....not the amazon visa....or something like a macys or kohls card) and the other category is non retail (like her AMEX, or any credit card that can be used anywhere and not just in one store chain).
I wouldnt expect her score to drop by losing her dads discover by more than a couple of points at first, but it will gain points at a much faster pace without it as long as she doesnt carry a balance on her card, and there isnt any balances on any that she is an AU on from your pile either.
Just my 2 cents
My understanding is that FICO dosen't care if you have Retail cards or not, and that the only place it may make a difference is if the retail card is the only one reporting a balance. As far as I know If your retail card the the only card reporting a balance it is the same as all zero. To maximize the credit mix you need both revolving and installment credit.
She is an AU on three of my accounts Chase Freedom ($3,000) and my Amex Everyday ($14,000) and Amex Gold. she was just approved for an Amex Cash magnet for $8,000 probably because of our good history with them through our Everyday and Gold cards. And these accounts were all opened under a year ago. So I feel that her AAoA will be hurt severely. We also don’t carry any balances on any other cards, we pay in full every month
To reinforce what others have said. With this profile she should have herself removed from her fathers account, and then after the normal report date for that account check her credit reports. If the account still shows have her dispute it as not her account so that it is removed from her CR. Make sure she does with all 3 CRA.