I would expect that 'dies to the ground' behavior in zone 6.
A shorter growing season could delay the start of blooming towards late summer/fall as Crape Myrtles tend to break dormancy late most years. They bloom on new wood and if your friend were to prune at the wrong time, it may further delay or prevent blooming altogether. It is best never to prune Crape Myrtles, except to remove dead wood of course. In zone 6, they should be grown as shrubs, which they are and not as standard trees. For a similar look, consider lilac trees, which are also much more winter hardy and can be grown in a Crape Myrtle-like tree form.
