What causes Wilt...

There are some questions that need to be answered to give an answer that is or may be correct.  First one is did she plant watermelons in the same place last year?  If so that could be the problem.  For one thing any plant that is not rotated will cause disease that is peculiar to that type of plant to build up in the soil and it can cause the plant to just die for no apparent reason.

Next and I suspect this may be the cause but I do not know for sure, had the melon patch just been fertilized?  Chemical fertilizers are notorious for causing plants to wilt if they are applied to heavy or to close to the plants they are supposed to help.  Organic fertilizers can also cause this problem if they are over applied or if manure is used and it is not composted.

Also did they cultivate just prior to the plants dying?  Watermelons and cantaloupe just cannot take close cultivation. 

The next thing that could have been the cause is proper watering.  Watermelons need to be kept moist, but never flooded.  They like a steady supply of water at all times.  Soaker hoses are good for this.  Never let the ground get completely dry around watermelons, but do not allow it to get soaking either if it can be helped. 

My dad raised Black diamond watermelons for over sixty years, and they are among my favorites.  They do have a tendency to "flatten" if they get just a little to ripe.  They are very sweet and succulent melons.

I hope this helps some. 

Happy Gardening and God bless,