Is this to big of an age difference...?

k my boyfreinds little brother is 17 and hes daiting a 14 year old he'll be 18 in september and she'll be 15 in december is this to bigof an age difference and what should his mom do shes threatening to send him to military school if he doesnt leave her. he refuses of coarse now there fighting and he's threatening to run away with her shoudl she just let him see her or what her parents are ok with it theyve only been daiting a few days. shes going to be reading the answers so that she can get an idea of what to do