To be blunt about it: your situation is simple. There are four people in the apartment and three of them want to have friends allowed over and quiet time to be later than 9pm. Because you have the numbers, you can easily give the ultimatum of get headphones or get out (whether that means study somewhere else or move out is up to her) and there’s very little she can do about it because you live in an apartment where the landlord doesn’t care as long as you follow the lease.
no you’re completely okay i understand the frustration! my roommate is the same way (as mentioned before obviously LMAO) and has a problem with the slightest raised voice/tv sound/anything outside of absolutely unreasonable quiet times. can’t even watch walking dead at a reasonable time (8:00) because if i do she’ll text me to turn it down even if i can’t anymore