Did you try the new Wella colour that uses a ppd substitute so is better tolerated?
If you’re just toning bleach highlights, maybe try a direct dye (pastel purple)? Obviously, you still need to do a patch test but clients are often more tolerant to direct dyes than full tints.
You can add a cheap white conditioner to a purple direct dye and mix them to get the tone you need. Put it on at the backwash as a conditioning treatment and don’t leave it too long.
It won’t last many washes so expect it to be re-applied every 4 shampoos or so. You can offer the client a bottle of your mixed colour for her to use at home between appts., just like a purple shampoo. Give her some single use gloves to use with it and charge a small fee to cover the costs of the bottle and product.
If she reacts to the direct dye, you need to be very honest with your client and don’t agree to put colour on using foils as it’s not safe and your insurer wouldn’t cover you if it went horribly wrong.