Their children are being advanced to a school system where they are expected to have achieved fluency in English.
The parent complaining is worried that his obviously English speaking children, will lose the Spanish they have learned over their elementary school years. I doubt the Spanish speaking students are going to worry about losing their Spanish. Is it possible that you are confusing dual language with an ESL program?
My #2 son speaks English and Spanish. As a real estate broker, he conducts almost 90% of his business with Spanish speakers. He represents them in court. His employees speak English, Spanish, Tagalog, Mandarin, Thai and Laotion. He views that as a competitive advantage. In exchange for that linguistic access, he mentors those employees to become more competent and productive in the real estate business.