My dad is interested in planting perennial ornamental grasses under a maple tree. I think Japanese forest grass might work, but I am not sure if they are hardy enough for a 5a zone. Any other suggestions?
Grasses for shade include Hakonechloa macra aureola and other Hakone grasses, Chasmanthium latifolium, and to some extent, Calamagrostis brachytricha.
That said, it is very difficult to grow anything under a maple because the fibrous root system sucks up all nutrients and water. I once was able to get Liriope spicata (not L. muscari) established under a silver maple. I suggest your father try that.
I have a Hakonechola macra aka Japanese Forest Grass growing under my Maple. It is about 6-8 feet from the trunk. I am in Zone 5B. Your dad may also want to try Luzula. I also understand that Karl Forester is good for dry shade.
The Karl Foerster I see around town (near Boston) is either beautifully stiff and full (in full sun) or thin, with erectile dysfunction (in shade).
All kinds of Carexes will grow in a dry shade and be pretty much contained.
In wet shade they are invasive.