The Beta series are not prototypes. In most electronics, the term "beta" means "second", not "prototype". The term "prototype" means "prototype". Thus, the Beta line is Shure's second generation of the SM series of mics. Generally, they are improvements over the original SM series, though as with anything in microphones, changes that improve the sound on some sources inherently make other sources sound worse, so YMMV.
The PG line, or "Performance Gear" is Shure's low end of mics. They are pretty similar in sound to the SM series, though they often are tubbier sounding. In a kick mic, I'm not sure a tubby sound would make much difference.
My advice, since this is a friend of yours, is to borrow the mic and try it. See how it sounds on your kit. If it works well, buy it and don't give it a second thought. If you don't like the sound, look into an AKG D112, as the Beta 52 probably won't sound dramatically better.
I'll tell you what
not to buy, though.
The Nady DM-90. When I replaced that with a D112... wow, what a difference. It's the difference between a poorly defined kick sound that got lost in the mix (Nady) and a clean sound that actually sounds like a kick (AKG).