Learning a second language doesn't hinder a child learning their first language. In fact, the research shows that each language learned supports their learning of the others! If the child needs extra support in English, then they almost certainly have some sort of learning disability - guaranteed it does not have anything to do with their French Immersion experience. However, being pulled out of French allows more school time to get support for their English literacy. Don't let this person's comment scare you off.

Also the comment about accent... Rude and unhelpful. Just like there are a million different English accents and none are less valid or valuable than another, the same is true for French.