So for context I’m a newish DM but I have a pc playing as a feral tiefling blood hunter (profane soul, hexblade) and the party has just reached 7th level. She’s a big fan of booming blade because why wouldn’t you be, I’m a big fan of the spell myself.

At 7th level she gets the Mystic Frenzy ability, meaning she can cast a cantrip as an action and then immediately attack as her bonus action. Very cool.

So as she often does she casts booming blade with her rite weapon (lightning) which as I ruled at the time resulted in: Action, booming blade; 1d8+Dex+2 piercing + 1d6 lightning + 1d8 thunder; Bonus action same again because booming blade ends when the target willing moves and she can attack as a bonus action after casting a cantrip.

This is exceptionally cool but I’m wondering if I’m ruling this correctly? Because this seems more efficient than just taking her standard attack action.

Booming blade doesn't augment normal attacks, it includes an attack as part of its casting. The effect should only occur once per casting.

•  Action: Cast booming blade cantrip
  • Make melee weapon attack
  • If hit, cantrip damage + weapon damage + crimson rite damage
  • Allows bonus action attack
• Bonus action: Melee weapon attack
  • If hit, weapon damage + crimson rite damage
• If the target moves, booming blade secondary damage

Sounds rite to me ;)

Ah okay, thanks for the clarification 

But does it also account for extra attack. So you get Booming. Bonus. Extra ? 

Booming Blade operates on the Cast a Spell action, which replaces the Attack(Extra attack) action.

Classes like Bladesinger Wizard have a special Extra Attack feature that allows them to replace one attack with a Cantrip. So,

(1) (Fighter with Extra Attack and Booming Blade) Take Attack action = Attack + Attack

(2) (Fighter with Extra Attack and Booming Blade) Cast a Spell action = Cast Booming Blade

(3) (Bladesinger Wizard with Extra Attack* and Booming Blade) Take Attack action = Attack + Attack OR Attack + Booming Blade

Quote from TylerMcgilvery >>

But does it also account for extra attack. So you get Booming. Bonus. Extra ? 

 What Memnosyne said. Casting a spell is not the attack action; and, the attack action does not let you cast spells (unless you are a level 6 bladesinger).