Brian Vogel <britechguy@...>
The easiest thing to do would be resetting your old iPhone to it's original "out of the box" state and letting her set up what she needs to herself.
I am unclear by what you mean by "still linked with my text messages" if you've transferred your SIM card to your new device.
Re-establishing your own Whatsapp account on the device should you need to later is not difficult, and you'd probably want to reset the phone again before you'd start using it again anyway (if that ever were to happen) as you'd want to be rid of all traces of her on it.
Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041
Always remember that computers are just glorified light bulbs - they rarely fail in continuous use and usually go pop when turned off and on.
~ Technician with the username Computer Bloke, on Technibble.com