I think OP so much wants this kid to be able to use Russian that little will sway her to consider more. But I’ll offer this.
When you’re talking about admissions to top colleges, many kids/parents think this is a special snowflake whose interests set him apart, whose courses, some charity or club he founded, that internship, programming some app, some EC so wonderful they won’t even mention it, etc,etc, etc, somehow exempt him. They think his “cream” will so obvious and desirable.
They don’t realize just how many kids are out there doing as much (including academic rigor and significant outside experiences) or more. The wise path is to play by the rules. Don’t give adcoms a reason to pause, to have that “Umm” moment. They move fast and don’t stop to hand out hall passes. At the schools with fierce competition they don’t have to.
The far safer choice is for this kid to take some high level Russian course, considering he is already certified proficient by UCLA, in addition to picking up the Spanish sequence online and ending with AP. Again, for a bright kid, it’s not that hard. Then adcoms would have something to nod their heads at, not something to question. And as a STEM kid, this could be a positive.
There are few shortcuts. Proceed with caution.