First, there's no such thing as a "gb", so you clearly have problems distinguishing upper and lower case, and it's thus unclear whether your client has 10Gb or 10GB of bandwidth -- and it makes a significant difference. However, even with 10GB, it's nowhere near enough. A minimal bitrate for well compressed HD video would be around 2Mbit/sec. That's 15MB per minute, so 10GB would let your client's users download about 11 hours of video. Except that if his data rate is for the aggregate of inbound and outbound traffic, rather than the higher of the two, we have to allow for the download from Amazon as well, so that's 5.5 hours. Basically, he can have two users download one movie each, and that's it for the month.
How to solve it: serve the video directly from Amazon, and pay them for the bandwidth. Bandwidth from Amazon costs 15 cents per GB, (less for high volumes), so that 10GB-worth of bandwidth would only cost $1.50 from Amazon. You'd end up paying around 40 cents a time for a movie to be downloaded. If he puts the links on his website, but they link to content hosted on Amazon, then the downloads won't go through his website.