Why is giving a teen too little freedom, basically none, bad?

My girlfriends mom barely lets her do anything ever. my girlfriend tries her best with everything and her mom just takes out her stress and because her life isnt good she doesnt want (my girlfriend) to be happy becuase she (mom) isnt happy. i prob now see my girlfriend once every 2 weeks for a couple hours. and its not just me. she hasnt hung out with any friends all year because any time her mom decided to let her do something, she spent it with me (few hours). My girlfriend tells me all the time that im the only thing that makes her happy. what can you tell me that will help convince her mother that too little freedom is bad? im not asking for a lot just maybe seeing my gf 2 times a week? why is giving a teen too little freedom bad??????? please help. i might print out what you say and give it to her mom if it sounds profesional and is convincing in order to convince her mom to lightin up!