she had to work and today she tould me that the only problem she knew about was when she was a pup she was having a hard time breathing during the winter
Boxers are highly prone to heat exhaustion but the only way to get some answers is to have the vet do a necrosopy which may or may not be possible.
In the heat any dog really does need to have ample shade and water at all times.
Why were you bringing the dogs out? Was she on holidays or at work? Could there have been an underlying medical problems that neither of you knew about? Neglect normally takes days, weeks or even months to kill the dogs - watch any resuce show and you see the horrible conditions that dogs have survived in for months.