We have a friend who has to use a walker. Is Wave Hill and its environs 'walker-friendly', so that where we can go she can go?

Yes it's fine but not all areas will be accessible. Part of the good stuff is behind a sloped lawn and /or narrow paths. The cafeteria is down a paved but steep road.

Yes

Pretty much. There are some steps and a few steep grades but it is largely accessible

I think so, however thete is a wooded trail and some steps in some garden areas.