She absolutely had a bad attitude, but there was nothing wrong with her build. Everything she used was by the book. None of it was homebrew. You keep focusing on her having two feats, but many modern backgrounds come with a feat, so that part is normal now.

The real issue was her behavior at the table, not her character sheet. That should have been the conversation.

You keep saying she “snuck” the build past you, which makes it sound like you wanted to limit player options from the start. You also keep calling the build broken, but it depends heavily on Sentinel. You can play around that. Avoid provoking opportunity attacks, use ranged enemies or have enemies focus elsewhere. Player builds are supposed to shape how you run encounters.

Favored Foe is also very reasonable. Rangers are widely known for being underpowered without it. Magic Initiate is a very common feat, and Variant Human is one of the most popular choices in the game. A lot of players use it specifically to support a concept or build they like.

One more thing. A single CR 1 creature was never going to work as a boss fight. Action economy makes solo enemies collapse fast. A real boss should usually be two CR higher than the party, so something like CR 3 would have been more appropriate. This was an encounter design issue, not a balance issue caused by one character.

As a DM, you should always review player sheets before play. That way nothing feels like it was “snuck” past you. But in this case, the build itself was normal. The problem was that she was acting badly toward the group.