The kennels should have insurance cover which could cover the cost of a postmortem, but the kennels should pay for one anyway.
It won't bring her back, but it would ease your daughter's mind to know what happened. I would ask for an independent vet to do a post mortem if it is still possible, although it is probably too late for this.
Such a shame but it in no way means people should be scared to board their dogs. When I was manager of a boarding kennels, we had three dogs die, one of a heart attack and two of old age, both regular boarders who we feel sure 'allowed' themselves to go whilst away from their owners. One of these died about two hours after the owner left it. They had checked with their vet and he was happy for the dog to go into kennels, as it was a regular and came in quite happily. Another had her dinner, then curled up in her bed and went to sleep, and when I went to take the dinner bowl out, she had died.
Tell the kennels your concerns, and I am sure they will discuss it with you.