Here is an remedy that was amazing for us and also is a perfect way to see if it is yeast. Mix liquid mylanta antacid and cornstarch into a paste and apply it to her bottom at each change. If the rash gets horribly worse in just a few hours you KNOW it is yeast because they will feed on the cornstarch and liquid mylanta. In that case, you switch to miconazole from the feminine supply area of the store - about five bucks for the generic kind. That will clear it up very fast if it is yeast. If it is NOT yeast then it is very likely an acid burn from the ph of the urine and feces and the liquid mylanta mixture will neutralize that better than anything else.
make double sure that the baby wipes are UNSCENTED - the other kind has fragrances taht are very hard on the skin. If at all possible, it is far better to use plain water on a soft paper towel (like Brawny NOT a store brand rough cheap towel - those are too rough on the baby's skin) or even a cotton cloth that you pitch or wash. (It is a GREAT way to get rid of your husband's old yucky stained tshirts - cut them up for disposable wipes and the menfolk can't dig them out of the trash to wear again, just a side benefit in my opinion.)
Depending on how sore she is, the momma may want to adjust the water in the faucet to where it isn't hot or cold and hust hold the baby's diaper area under the running water - it is better than even using plain water on a soft cloth - but you must be sure the water temp won't spike up.
Jess had the WORST diaper rash problems and a TON of yeast infections there and that mix of cornstarch/antacid was the fastest way to figure out what was causing it. You will make a yeast infection worse but then won't spend several days trying this and that remedy to figure out is it is yeast or not. For me, the fast route to knowing and proper treatment got my kid out of pain fastest even if for a short time the yeast flared badly.
Yogurt is fine to apply for a yeast infection, but you will get faster results from any of the OTC creams for yeast infections. I always got the cheapest one that did not come in pre-filled applicators - either miconazole or clotrimazole, didn't make a difference for us.
If this is a persistent yeast infection then the cause needs to be found. It could be not changing diapers often enough, not using a wipe when changing (for some stupid reason we had several people at the daycare J went to who thought it was fine to just take off the wet diaper and put on a dry one with-o wiping them off, but this tears up the skin super bad super quick.), an immune system issue, even a diet issue. I know quite a few kids who stopped getting nasty rashes when switched to soy formula or when mom cut out most dairy from her diet if they were nursing.
One function of diaper ointment is to provide a barrier between the child and the diaper. I couldn't use desitin because it was so hard to spread and it seemed to hurt when my kids had rashes. One of the best things we tried, after eliminating yeast as the problem, was to use an ointment or lotion with dimethicone. Gerber used to make a diaper ointment with this years ago, but I think they pulled it. I used the Monistat Chafe Relief ointment on my niece and it was awesome. The dimethicone is a type of silicone that will NOT hurt the skin and it stays in place and is a better barrier than the thick layers of cream that get wiped away. You don't need nearly as thick a layer. At one point I was out of the diaper goop with dimethicone and I grabbed Gloves in a Bottle hand lotion. It is one that is designed to stay in place even if you wash your hands for at least one handwashing anyway. The Gloves in a Bottle worked as well or better than the diaper cream with dimethicone because it also moisturized the sore tissue and helped soothe it.
Aloe vera gel can also help. Be SUPER careful to read the labels on any you buy. most of the ones sold for after tanning remedies have alcohol in tehm and it BURNS terribly. It is far better to stick to aloe straight from the plant.
Parents also MUST look at any changes in the diet - they actually should keep a log if it is an ongoing problem. One of the ways we diagnosis'd thank you's food allergies was by the hideous diaper rash he got after eating those foods.
If the baby bleeds or the rash doesn't clear up after a week or so, the pediatrician needs to know.