It is a tricky one, actually. Dd1 (just 11) has been wanting to do a German exchange for a while, but we haven't really been able to get it off the ground because there are so few people keen to do this kind of thing for primary-age children. Allef in JanH's link are a very reputable outfit and seem very nice (I've had several conversations with them) but only operate exchanges for a minimum of 6 months each way (ie. your child goes to France for 6 months, French child comes to you for 6 months) which seemed a bit too extreme for most tastes, and could wreak havoc with English secondary transfer system.
Mill on the Brue are very good pastorally (it's one of their strengths), unlike some summer camps which can be a bit sink or swim. Dd1 went for the first time aged 8 not knowing anyone else there, and had a ball. But I think it is quite a tall order for a non-English child unless they are very robust or already have pretty high level English. If I was seriously going to send my child as a non-fluent English speaker, I'd probably want him/her to be accompanied by a friend to make the shock less extreme.
Is your friend's child a boy or a girl? Ds (nearly 7) would probably be up for it, and frankly for me one extra child more or less isn't a big difference. But 'Oh, it's someone I've encountered a few times on an internet chat board' is probably not what they're looking for by way of a recommendation...
Alternatively, some boarding prep schools take non-English children for a term. That would probably be my preferred option for a serious language learning experience, but you'd need a pretty robust confident 8yo to be up for it.