My youngest brother goes to an Islamic school. I think its the best decision my parents made for him especially living in U.S & whenever I have kids of my own I am gonna do the same InshaAllah! They get to learn what they will learn in regular school & they also get to learn religion & sometimes even our culture at the same time.
Niksik if the school she mentioned has a good curriculum & high rankings too then I don't think your friend should be worried about the communication part. If the Islamic school is board certified then there shouldn't be any worries at all. Its not a "madrassa" its a proper school. He is gonna be learning "deeni" things and "duniyawi" things. Yeah do make sure that the school has the same thoughts when it comes to religious preferences as I am assuming that is the major reason she is sending her child to an Islamic school.
My brother went to a public school till first grade & then switched to an Islamic school. His vocabulary increased by 70% in the first year & I am pretty sure about that one. I think the education standard is much better than the public schools too. Most of the times private schools have a good teacher to student ratio. In the start I was the one who critized this decision but now when I see other kids his age who go to public schools I see a big difference in his attitude & the way he treats others. I actually find him to be more confident than other kids & on top of it he gets to learn namaz, duas, kalmas & also Islamic stories from back in the days. He is really good in maths & regularly takes part in extra curricular activities too.
The only down part is that his school doesn't have a proper gym. My siblings try to make that up for him by taking him to parks or lake 2,3 times a week. Other than that there are times when he comes home & sometimes mentions things that we think are a bit too extreme. That's when we always talk to him & explain to him every thing we know Islamically. I find his school to be a bit more strict when it comes to religious thoughts. But then as parents our job doesn't end by sending the kid to school. We have to interact with them & teach them things as well.
If she has the opportunity & can afford sending the child to an Islamic school I say she should go for it. Also she can also talk to other children's parents & get an idea on how the school is.