Why not use Altair (the first-magnitude star in the constellation Aquila)? It's very bright and conspicuous, and it's high enough in the eastern sky in the early evening hours of July to be spotted easily (in fact, it will be obvious all night, if you want Faramir and his granddad to stay up late after all). And Aquila means "eagle"; Tolkien conveniently gave us a constellation name Soronume, which also means "eagle", and could plausible be associated with the very conspicuous constellation of Aquila. The stellar Eagle of Manwe, perhaps?

Other possibilities: Vega (in Lyra), Deneb (in Cygnus), Regulus (in Leo), Spica (in Virgo), Arcturus (in Bootes), Alphecca (in Corona Borealis), Antares (in Scorpius). All would be easily visible in the early evening hours in July. Constellations that lack a single prominent star, but are easily recognizable include Sagittarius, Cassiopeiea, and Ursa Major (the Big Dipper) and Ursa Minor (the Little Dipper). I'm assuming you want fairly obvious constellations and stars, not faint ones you have to really work hard to see. If you can do faint constellations in the story, the list grows even longer!

Hope this helps! Let me know if you've got other questions I can help you with as you plan the story