I did some work experience way back when in a London primary school that had a Muslim majority. They were honestly the kindest, sweetest, most gentle children I have ever had the pleasure of meeting. I cannot even accurately put into words how downright beautiful these children were. They were all just such lovely little souls. I've never met such well mannered children and they had so much respect for their peers and teaching staff.
I'm naturally very pale - think vampire pale. And they were all fascinated by my colour. They would put there arms next to mine to compare every morning and say, "Still no tan Miss Vino!" 
Personally speaking, it wouldn't matter to me an ounce what the majority was in my own dd's class. As long as she is surrounded by pleasant children who she enjoys being friends with and she ticks along at school just fine, then there is no problem. Why move a happy child?
The problem with adults is we don't see the world through a child's eyes anymore. Whilst your friends may see a minority as 'a problem', their children won't distinguish race, skin colour or religion as anything other than a simple, inconsequential difference within their friendship group. And unless their child is somehow being excluded as a result of these differences, I'm not sure I see where the problem is 
When my dd makes a new friend and I ask her to tell me all about them, she doesn't start off by saying: "Well, she's black and a devout Christian and she goes to church every Sunday and she says her prayers every night and..." She doesn't see any of that. Nor would she care.
No. My dd's response would be the same as most children's: "Oh, she's really nice and we played together at lunchtime and oh Mummy, she loves Monster High too, just like me! Can she come for a play date if I tidy my room, pleeeeeeease?!"
I think your friends are being massively shortsighted, and it's a real shame.