That's sad, I feel badly for your neighbor. Boxers should not run long distances until at least 16 months of age. Their joints and bones are not ready for that much work and there is a potential for long term joint problems. Also, Boxers are sensitive to heat so if it was a very long run she may also have overheated. Boxers also have a tendency to have cardiomyopathy, it's becomign worse and worse in the breed due to BYB so she could have had a heart problem as well. In any event that is very sad.