We always advise our customers to carry out a patch test prior to any treatment however, allergies can develop at anytime. This means even though your clients patch test came back negative, they could have developed a reaction between the time of the patch test and the time of the treatment.
In the event your client does have an allergic reaction, advise them to apply a cold compress to the affected area. If the reaction does not get better or worsens, they should seek medical advice.
Please note most insurance companies require a patch test so it is vital these are not skipped.