I'm not familiar with the BMW stuff, but I dont' think any of thsoe will meet his criterea.
Formula Vee's have enough power to hang it out. The power to weight ratio is very much where it needs to be. Let him learn a little bit and push him to do that, but don't force your opinion of what he should buy. It is, after all, going to be his car. Nothing is wrong with the F bodies that are well set up.
There's quite a big community of Formula Vee'ers out there that can help him out.
Another "more substantial" vehicle is a lola/formula ford. From what I recall, those are beyond the $15k budget
Or if he wants to go really fast for cheap, shifter kart. One could find and build a nationally top competative kart for $5k, massively cool acceleration, braking and handling, massively cheap parts, easy to work on (20 minutes to rebuild the motor). The most liberal motors are ~40 hp and the most conservative are 32. Minimum weight would be 385lbs without driver.
Can you tell what I would like to be driving yet?