What should i do about my friends parents hating me?

This year i met an exchange student from japan and we became close friends. I have never in my life met or even talked to her parents (not her real parents...just who shes living with) but because im friends with this girl they dont like...they instantly dont like me?! Now shes going back to japan and i decided to throw her a farewell part...im planning it, paying for it...EVERYTHING but when she asked if she could go they said "you can go but ashley cant" (ashley is my name) umm wtf im pissed about this! who are they to tell me were i can and cant go..WHEN IM PAYING AND PLANNING EVERYTHING! If i go, she cant....uhg what can i do to convince her parents to let this go on without any problems?!