Removing phone number (payment method) from account/device I no longer have access to

What you say implies you are using mobile billing, and this isn't on your account, it is on their account. In that case the suggestion to remove the method from your account would not apply. It would still be on their account.

You could just let her subscriptions "bounce" in terms of payment if the balance is low. You would, however, need to change the payment method on your account so payments you need to make would go through.

I don't think contacting Apple will do anything. Frankly what you have happening if a person is using your payment method on their account without permission is fraud. Apple doesn't handle that. You do not have access to their account to request removal and Apple won't do it for you unless you have full legitimate access to that account. This being mobile billing add complications since I am not in a country where that is allowed and have no firsthand knowledge of how it would go. You either need to contact your phone company which is handling the billing using your number, or your financial institution acting as the basis for the payment.

If you are using the same payment method for your Apple ID, you should change at least your payment method, as all Apple Services will be stopped, if there are unpaid bills.

Remove a payment method from your Apple ID - Apple Support

Only the official Apple Support might be able to help, you can use this link as a start:

Contact Apple Support

I have little familiarity with how mobile billing works since it is not a common feature and is only allowed in a a few countries. My impressions is she would be the one who would have problems if your number could not be charged since she is trying to use it from her account. Again, all I know about mobile billing is what Apple says in its support documentation which is very little. Is there a password or code on the service that you could change, essentially preventing her from using it? Frankly if all it requires is a telephone number then that's essentially zero security. There must be a way you can disable her from using it, but I'm afraid you will have to find out how to do that since it is not information available to me.