Top 10 songs? I have to agree with Angie, most anything by those guys is good, but I have my favorites.
Reba: Fancy, The Night the Lights went Out in Georgia, Secret, Go Tell it On the Mountain, I'm a Survivor, She Thinks His Name was John, One Honest Heart, Forever Love
Tim McGraw: Let's Make Love, Indian Outlaw, Don't Take the Girl, It's Your Love, I Like It, I Love It, Please Remember Me, Live Like You Were Dying
Shania: It's hard to find a song of hers that I don't like, I will say tho on her UP CD, I prefer the more Pop sounding version rather than the acoustical.
There are more, but these are the ones that came to mind for me. Maybe someone else can think of others. Good Luck and Happy Birthday to your friend.