Rather tricky set of criteria, so he WOULD be coming over the bridge. Tenafly is almost directly across the river from Yonkers..... Ok, can't speak for the Jersey side of things, but after crossing the bridge he will definitely have to ride in considerable traffic to get through to the top portion of Manhattan.

Hmm, 70 pound electric bike. Sorry, but I would definitely advise avoiding Inwood Hill Park. It is kind of a round about way to get to Hastings unless you think you can actually get the bike through to the approach to the Henry Hudson Bridge. Actually, with that sort of bike, once you are in the Bronx, I would probably avoid the Old Putnam Trail in Van Courtland Park as well. The OPT turns into the South County Trail, but can be muddy if there has been any rain recently (there are other threads on this trail). It is paved once it actually turns into the South County Trail.

The most direct route is to come over the bridge and then stay on Broadway which turns into 9 all the way to Hastings. Not the safest in terms of traffic or the most scenic. I think the best mix might be B'way to 240th in the Bronx, then into Van Courtland parrallel to B'way until the park ends, then back onto B'way until Yonkers and then the Old Croton Trailway. You can pick that up slightly north of Ashburton Avenue in Yonkers.