Well, drainage is good ................................. but how big are the lumps of cement and blocks etc for a start?
And how much depth of compost will you have?
And then, what I suspect you're most worried about, we have the lime content of the cement ......not being a camellia grower I can't really help you there, except to say that my parents had a very healthy camellia (Donation if I remember correctly) in a bed surrounded by concrete paths, and I have seen others in similar situations, and I've discovered this article on the internet http://www.camellia.org.au/UserFiles/file/Lime%20&%20Camellias.pdf which implies that it ought not to be a problem.
However, I think the knub of the question for me is that you would know the stuff is there, it's not how you think the bed should be constructed, and I've a feeling that a certain brother-in-law has just chucked it in there because it's easier than taking it to the tip and disposing of it properly.
If you want it done properly you can try telling him that Dove on the GW website has recommended that the base of the bed should be filled with a layer of crushed and graded hardcore mixed with gravel no deeper than one eighth of the depth of the bed - the rest of the bed should be of good ericaceous compost - and then stand well back - I'm wearing a hard hat for the rest of the day