Dear,
Welcome to HCM
We understand your concerns
I went through your details. First of all you should confirm they are rashes. A genital rash is a skin symptom that can be caused by a number of health problems and can occur on any part of the male or female genital area. Rashes are normally reddish in color, may be painful or itchy, and may include bumps or sores.
From the given details, This sounds like irritation due to friction, nothing more. This does not suggest
allergy in any way, just mechanical irritation. I suggest moisturizing before and after sex, lubrication during sexual intercourse. Your fiance's skin is irritated, not damaged or having incurable skin disease. For any further advice, you'll need to see a doctor in person.
If you require more of my help in this aspect, please use this URL. http://goo.gl/aYW2pR. Make sure that you include every minute detail possible.
Hope this answers your query. Further clarifications are welcome.
Good luck. Happy New Year. Take care.