I've got to agree with the other who said go slow.
But I have a couple of ideas to add.
At least near me there are several streets with decent bike lanes, or in some cases wide right lanes with areas where parking is legal and close to never used. Such streets give a chance to ride a road with cars, but also with distance from those cars.
Also one can feel safer in a pack. I'm in the Los Angeles area and there is a beach bike path that runs from North of the Santa Monica Pier to Palos Verdes. Over 20 miles. There are at least 3 sections that dump onto the street. But if you ride it on a weekend you are apt to be in a pack on at least 2 of them (Especially Washington Blvd.) Something like that might help. (But do not surprise her with the street sections, avoid nasty surprises like the plague).
Thinking of the beach path brings up another issue. riding the beach is pleasant for most people. riding a city street or even a city path often is not all that nice until bitten by the biking bug. Also when I used to ride that path a bit I would drive to the beach and then park a half mile away, for free, and ride the bike path. I'm sure beach parking is $10 or more by now.
Try to find a route with at least some sections she really likes. The not so nice sections go so much better when you know there is a good part to come.