The 520 trail is not that scenic. It does parallel highway 520 the entire way, and parts of it are on roads (with recently added bike lanes). It's more of a commuter route rather than scenic route. But from where you are staying it's the most direct route.

Your route around on the north end of the lake on the Burke-Gillman and Sammamish River trails is a very nice route that you will enjoy. If you don't mind the extra mileage, I would do an out and back on this route.

Other advice/option I would add to your route:

- Where the Sammamish River trail hits Leary Way, cross the Sammamish River and rather than staying on the Sammamish River trail take the 520 trail up the hill to the Microsoft Campus.

- An alternative route back to Seattle that is a bit more scenic: from Microsoft Campus head back down to West Lake Sammamish Pkwy. Head south and make sure to stay on W. Lk. Sammamish Pkwy. where is splits at Bel-Red. Rd. Ride along the lake, cross under I-90 and turn right onto Newport Way. turn right on 150th and left on SE 36th St. At the bottom of the hill where SE 36th T's with Factoria Blvd., stay in the left lane and across Factoria on the left you'll see the entrance to the I-90 trail. Take that across Mercer Island and the I-90 floating bridge into Seattle.

See the Strava Global Heatmap for routes that cyclists ride in the area. There are a lot of hills and roads not suited for cyclists.

An aside, not sure exactly what your friend is expecting on Microsoft campus. It's a bunch of commercial buildings with some open space thrown in here and there. Well kept for sure but not especially scenic.