First take her to the doctor to make sure she has not gotten an infection. Second wash her good every time you change her diaper, make sure you have dried her good before putting another clean diaper. I used to put on my babies some baby oil after I dried them good, this prevents the acid from the urine to get to their skin. Her diaper should be changed every time it gets wet or soiled. I know this may sound old fashion, but we all survive in the old times with these remedies, better than with what they have today.
Lord heal this baby's diaper rash, teach and give wisdom to this new mother on how to care for her baby. I pray in Jesus' name amen.