What shall v do when a person becomez obsessed wid us?

1 f moi really close palz is facin a unique probz... 1 f hiz clasmatez has a serious crush on him... hez a happily committed person n is least interested in flirtin arnd wid oder galz... so he declined her offer 2 get 2geder... bt diz gal has been actin strangely since den... she sayz she givez a damn if hez committed o nt, she jus wantz him... havin him in her lyf has b sum kind of obsession 2 her!!! she cant stand it if he evn talkz 2 ne odr gal n if he ignorez her, she behavez really awkwardly like starin widout blinkin, cryin 4 hrz 2geder, even pullin d guy out f d group hez talkin 2!!! d gal lost her father when she waz quite young n belong 2 a rich family so all her needz n fancies hv been fulfilled all her lyf!!! v hv tried talkin 2 her bt shez nt ready 2 listen 2 nethin... d only answer she givz is, i jus want him, m gonna marry him!!! hw do v make her understand dat evrythin wish f man dsnt come true??? her xtreme nature also scares us dat she mite take an xtreme step if v pushed her 2 d limits of ignorin!!! moi frend has started stayin tense since diz incident n m quite worried abt him... plz hlp us out f diz situation!!! moi frend lovz hiz gf a lot n dsnt wanna loose her at ne cost!!! can ne1 hlp us??? plzzzz... :S