I'm a new DM and I have a level 6 ranger in my party. In our game, we came across 4 closely packed enemies in a 1×4 line. The ranger attacked twice with her short-sword, and twice with her crossbow. She has an Extra Attack, Horde Breaker (class feat), and Crossbow Expert.

• Action: attack knight #2 with crossbow
• Extra Attack: moved in and hit knight #1 with short-sword
• Crossbow Expert: hit knight #4 with crossbow (because she hit knight #2 with short-sword)
• Horde Breaker: short-sword attack against knight #3 who was within 5 feet of knight #2 who was original target.

She also has the Mobile feat, so didn't provoke any opportunity attacks from the enemies as she moved, attacked, and then moved away. I should also note that in our game I edited the Mobile feat to include any attack (and not just melee) to make it interesting for caster classes to take, which I acknowledge makes it an incredibly strong feat.

After the game, one of the other players started saying that she couldn't do what she did, saying that she can invoke either the Mobile feat or the Crossbow Expert feat "since a feat is like a bonus action but without the bonus action penalty... you can only use 1 feat per round."

I've never seen anything saying you can only use 1 feat per round... Crossbow Expert expressly says it is a bonus action, but Mobile doesn't say that, it just says any enemy you attack no longer has the chance to get an opportunity attack against you. Am I wrong here?