I cannot confirm anything without examination but it can be eczema,diaper rash(in infants),dermatitis or sweat dermatitis.
Wash the areas several times with fresh water. Do not use any cosmetic products at the sites. You can apply some calamine lotion at the rash as it will help in soothing the skin.
Also use good quality moisturizers to prevent moisture loss from the skin.
If still the symptoms do not improve then pls get a clinical examination done by a dermatologist.Steroids may be needed which are available under prescription.
I hope that helps.Take care and pls do keep me posted on how your fiancé is doing or if you have any additional queries.Kind regards.