In a 10 year old yes I can understand concern but not a 5 year old. This is exactly the age when kids are starting to be more aware of the world around them and come out with inappropriate things.It's our job to put them on the right track.
Jesus in an average class of 5 year olds you'll hear all sorts of derogatory comments re all sorts of things.At this age kids need to be told which are inappropriate and educated accordingly.
Ringing a parent would be a strategy far up the list ie is she really unable to handle it herself,slightly concerning if she isn't.
Op get him a load of picture books with characters from different ethnic groups,explain how we're all different and talk things through if he makes any other ill advised comments, then just leave them to it. If the mother wants to blow it up out of all proportion there isn't much you can do. Within a year he'll have learnt what is appropriate and what isn't.
This time last year my curious 5 dd asked lots of questions re"brown people"
and children with disabilities.She's got several black dolls (and books featuring different ethnic groups) one she chose as she loved it's "puffball hair"
.We had many a conversation about phrasing things correctly l and how we're all the same but different. Now she knows exactly what is right and wrong in terms of language re race,commenting on differences with people etc.
5 year olds just haven't learnt political correctness at this age,some may if surrounded by cultural diversity others won't. Like all things some children will need more support (gentle discussion and educating).By KS2 most will have learnt.
I think the way this teacher has handled this could well cause a lot more damage than gently explaining why it was wrong.