Agree with Tom's suggestions, or if your friend wants something a bit larger maybe a red horsechestnut or pink magnolia?

There aren't many medium sized trees hardy to zone 6 that have conspicuous flowers in colors other then white. I think serviceberry are too small to be a shade tree.

Purple leaf plum? Flowering cherry? The latter are common but temperamental. Magnolia and crabapple are good choices.

Does the ornamental feature have to be the flowers? American holly have pretty red berries.

If you can put up with white flowers, there are yellowood, fringetree and sourwood.

Are there cedar or Japanese beetles in your area? What threats would this tree face? What is the soil like? Sand or clay, acidic or basic, is it ever saturated?

My generic acer rubrum put on a good show early this spring with red flowers and samaras. Pick the right cultivar and you'll get good fall color also. No individual flower is big so it took until the tree got some size for me to notice the overall effect.

• This one would probably be a little too small and a little too slow for your friend, but I'd be inclined to give the native Aesculus pavia a long look. Mine was a gift from a friend in 2012 that is just hanging on at the dry woodland edge. I really need to move it to a better site.
• It needs moist soil, but panics in wet conditions. It must have adequate sun, but sulks if it gets too hot. Has been known to live in cold conditions, but I don't believe it's much better than z5, poor thing. It's a diva plant, but oh those flowers. :)

Agree with the Aesculus suggestions. Like Maackia says-a bit finicky-but truly splendid during bloom period, and not a bad plant the rest of the year.