The woodchuck is typically considered to have a bimodal activity pattern, being most active in the early morning and late afternoon. It is not well adapted to nocturnal activity. However, many "diurnal" animals are often active at night, especially when there is some ambient light (moon- or artificial). If your friends have seen woodchucks after dark, there was at least enough light for them to do so, and humans aren't well adapted to nocturnal activity, either.
