Ours private school starts making noises about children possibly not passing the entrance exam in Yr5 too, a sort of pre warning to parents, who may start tutoring, or look elsewhere, there is another private that is known for taking kids on from ours who parents want to move them (because they aren't going to get in the seniors). Usually one or two from 40 or so children.
Personally I think if your child has been there from Reception and has got to Y5 and is struggling, then a lot of onus should be on the school to take responsibility and do something, or have intervened beforehand and put help in place to prevent this. But the fact remains, some schools only want the highest achievers, they don't care about the money you have invested over the last 7 years for a good education, but tht is a whole different debate!
As Millymomma says, maybe a different school, with less pressure may be better for him, he would probably get more support in a good state school, unless the private has good SEN provision.
Has he a learning difficulty that hasn't been identified, how long has he been there and why did he move around schools?, which is disruptive in itself.
Lots of private schools are happy to put children up or down a year, but not necessarily repeat a year, unless through illness.
I think your friend needs to speak to the school and ask them questions on what the difficulty is, do they repeat years etc. They may just be being polite and giving them the option of finding alternatives in plenty of time. If he has only been there a year or two, they may not be as understanding as if they have had him from the start.