For ZippyPinHead....

Zippy, best I can tell, the recommended way to reduce the level of radon gas in a house is to run pipes underneath the slab foundation and then via fans suck the radon gas out  from the soil underneath (depressurization).  It should be vented to a place well away from the house, definitely away from windows, where it can be disbursed into the atmosphere.  This is not a one time thing, but needs to be running permanently.  Just sucking it out from the soil underneath once and then sealing holes in the slab, such as around plumbing pipes, has NOT proven to be effective as the gas comes right back.  This will need to be an ongoing effort. 

For your reading pleasure:

According to the EPA's "A Citizen's Guide to Radon",^[11] the method to reduce radon "primarily used is a vent pipe system and fan, which pulls radon from beneath the house and vents it to the outside", which is also called sub-slab depressurization, active soil depressurization, or soil suction. Generally indoor radon can be mitigated by sub-slab depressurization and exhausting such radon-laden air to the outdoors, away from windows and other building openings.^[12] "EPA generally recommends methods which prevent the entry of radon. Soil suction, for example, prevents radon from entering your home by drawing the radon from below the home and venting it through a pipe, or pipes, to the air above the home where it is quickly diluted" and "EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently" according to the EPA's "Consumer's Guide to Radon Reduction: How to fix your home"

Something else to consider:  This problem, even if mitigated, will need to be disclosed at time of re-sale, and will likely have a very negative effect on value.  Your friend might check with the homebuilders association there in AL to see what experience they've had with this issue and what remediation they might recommend, and maybe even ask if they might be able to recommend an experienced contractor.

If I find anything else that might help I'll pass it on here as an edit.  

Good luck.

S

Also check here:

http://www.forensic-applications.com/radon/radon.html

https://www.rdsenvironmental.com/radon-mitigation/radon-mitigation-basement-and-slab-on-grade/