Idk if it'll work the same for your friend, but when I was on the fence, the first episode my brother showed me was The Puppet Master (Hama and the intro to blood bending) and I thought it was great. Personally, I loved Book 1 and think it's a good starting point but I can understand the hesitance since the build is a little slow. I like Puppet Master because you got to see the morally ambiguous use of violence, it's kinda spooky, and it doesn't spoil any big plot points imo.
Another good one I think would be Avatar Day. Kyoshi popping up and saying "I killed Chin the Conqueror" is one of my fave moments. Idk, I'm probably being so biased and unhelpful so I'll stop rambling :'D
I mean, you could definitely start with Korra too, but they'll lose so many great references and they'll end up watching it again anyway after ATLA to revisit the references :P
I'm a fellow water bender, but I also think earth would be a lot of fun. Southern Water Tribe forever <3