...Whoever that elf's friend is better be quite the character, because picking up a million contractions ("y'see"), idioms ("pumped up") and mannerisms (using "like," every few words) is not something you see on someone learning a language. It's something you'd see on a native that's already intimately familiar and comfortable with a regional dialect.
As it stands the dissonance between what's being presented and what it purports to represent is annoying and immersion-breaking.
And if that friend turns out to be a scholar of some kind instead of some redneck or whatever that found her in the forest I'm going to scream.
As an ESL, I've definitely seen friends who were much better at vernacular English than formal English. I just had the good luck of preferring to read stuff, where things are usually more formally written, but those who learned languages by immersion would find vernacular language to be the 'default' state.
Heck, I still don't understand
keigo but can speak like a Tokyoite because listening to streams and radio is where I learned most of my spoken Japanese.
Not saying you're wrong of course, I think the elf being in academia should prime her more to Wikipedia-sounding speech patterns instead of fully casual language like that, but I think it's also not impossible to pick up street-speak before the proper formal form of a language.