Girl has completed 18 or short of few days on the day of eloping with her adult boyfriend? No crime if eloped on completion of 18 years age. Both can live together even without marriage, provided the girl over 18 doesn't Lodge FIR of rape. Consult some local lawyer. The girl's parents may have mentioned her age below 18, hence the FIR. Do you have her DOB certificate from Municipality, gram Panchayat or high school?