Is the engagement ring 9 or 18ct? Is the whole of the wedding ring discoloured or just part of it? Could it be that your customer's skin is reacting with the 9ct, which of course is two thirds base metal after all! I had a case where the acidity in the skin of a customer meant she came out in a rash while wearing 9ct and the gold went a yucky colour as she was reacting with the base metal part of the alloy. 18ct and above was fine, which although it was a blow to her husband was good news for our till!
Of course it could also be that the rhodium plating is coming off the white gold part ( assuming you had that bit plated?)