If you have success with one of those el-cheapos I'll have something concrete to recommend to him.
So, as usual, there's a couple of issues that are possible with these cheap Chinese products…
None of the inexpensive (read: cheap) Chinese BDA's appear to have an FCC type certification (raise your hand if you are surprised). Like their cheap radios, the often don't meet the requirements to be sold in the USA, but that doesn't stop anyone who wants to make a quick buck.
Since -some- public safety and cellular frequencies are quite close, using a non-type accepted amplifier runs the risk of creating interference to a public safety radio system. That won't end well. Putting others lives at risk to save money is
always a bad idea.
Proper BDA systems need to have a way to tell when they are "self oscillating" and shut themselves down. Good ones/type accepted ones, will have this built in. What happens is that if the donor antenna and the indoor antenna are too close (or can "see" each other), it can create a feedback loop, like getting a microphone too close to the speakers. A self oscillating BDA running unchecked can wipe out the cell site. That would impact everyone on it, not to mention any possible public safety users, other systems, etc.
The other big issue is that consumers don't always remember to read the directions. If these are set up incorrectly, they can cause all kinds of issues (see above) or just not work at all. There's some work that has to go into setting them up correctly. Using the correct antennas is a good idea. Where possible, directional antennas should be used on the donor side.
By FCC rules, these BDA's should be registered with the carrier. I'm will to bet that rarely happens.
I'd probably avoid buying one of these things, just like I'd avoid buying the Cheap Chinese Radios. Too much risk, cheap product, supports a country that doesn't value it's work force, doesn't meet regulations, etc. The FCC/public safety users won't care that you got a great deal.
BDA's can really suck.
If possible, get the internet connected pico cell from the carrier. Much better solution and can actually work better.