About allowing the Rogue sub-class (Assassin) to benefit from Supreme Sneak


I have a player in my campaign who plays as an Assassin Rogue and would like to have access to the ***Supreme Sneak*** upgrade at level 9, which normally is part of the Thief build. 

Her argument is that *Supreme Sneak* is more appropriate to an Assassin because of its synergy with *Assassinate*. I can see her point and I don't have any problems with allowing this, but I wanted to know if there are any glaring problems with doing this.

#Would allowing a Rogue (Assassin) to benefit from Supreme Attack upon reaching level 9, instead of the regular Infiltrator Expertise, make the sub-class very unbalanced? 

I appreciate your expertise with this. 


> **Assassinate** (Assassin)
>
> Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.

> **Supreme Sneak** (Thief)
>
> Starting at 9th level, you have advantage on a Dexterity (Stealth) check if you move no more than half your speed on the same turn.
>
> (PHB p.97)