You need to be extremely cautious if the client has already told you that she is allergic to hair colour. Your insurance company may refuse to cover you in the event of a claim so give them a call to check your position.
Basic skin test: Mix a small amount of the tint you want to use (but a darker shade) with the same level of peroxide you will be using and apply a dab behind her ear and wait for at least 48 hours before considering a full tint application.
Obviously, if she starts reacting to it, remove it immediately.
Even if she's having foils, you MUST do this.
Severe reactions to tint although rare, can be fatal! As you already know your client has some sensitivity, exercise due care.
She could try using semi-permanant hair colour as this is usually less chemically toxic, but a skin test will still be a sensible precaution.
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Just re-read this and it probably sounds a bit OTT. However, as I suffer from allergies to some chemicals, I'm probably a bit more cautious the most. I realise that as a pro, you already know about skin testing but as this is the start of the academic year, there might be a larger proportion of newby geeks who could do with the reminder... ;-)
Hope you can find a suitable alternative for her. Aveda is supposed to be good and there are a few other pro ranges that might be ok.