It is extremely rare that a person would be allergic to an essential oil because there are no protein molecules in a pure essential oil, according to Dr. David Stewart in The Chemistry of Essential Oils Made Simple.  It is more likely that the person is experiencing a detox response or that the essential oils or product contains additional substances that are not 100% plant based.  Normally, drinking more water and cleansing the colon can eliminate annoying responses.  However, if there is a rash, it may be better to apply  oils on other parts of the body rather than directly over the rash.