Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your daughter has had so much trouble getting her pup to eliminate outside on the grass.
Part of housebreaking is teaching a dog where you don't want them to eliminate.
But what people don't realize is that for the dog where to eliminate is really the most critical part of housebreaking rather than where not to eliminate.
Most dogs have strong desire not to soil their den, but beyond that they identify where to eliminate by scent (the smell of their urine or stool in places where they have previously eliminated) and by surface preference.
Very confident dogs will eliminate in places that have the odor of any dog having eliminated there. However submissive dogs will avoid voiding where other dogs have eliminated. They are not comfortable "marking" over the scents of other, probably more dominant animals.
Also in the training process a dog will often develop a particular surface preference for where they like to eliminate. It may be on grass, pebbles, cement or other hard surfaces. Once a particular area and surface is identified as appropriate for some dogs they may refuse to eliminate anywhere else. Some will hold their eliminations for hours waiting to eliminate where they have deemed appropriate. That's helpful in keeping the home clean, but can make it very difficult when traveling.
It is helpful in these situations to enlist a buddy dog in the task of retraining. Dogs learn best by observation of a trusted dog friend. This can take a long period of time, but it can usually be accomplished. You can hasten the training by putting a piece of the favored surface (plastic, fake grass) outdoors on the grass and taking him there to eliminate when you know he needs to go. And of course praising profusely and favored treats to reinforce the wanted behavior.
To help you understand how he feels it would be like your going into a restroom but being told that you must eliminate on the floor or matt in the restroom rather than the toilet. While you are essentially in the right place, the specific place/area is wrong.
Please let me know if you have any further questions.