There's two versions of this flash - the standard EF500 DG, and the EF500 DG Super. The Super has a couple of advantages, including wireless operation and flash compensation. With the standard DG version you don't get wireless, and there's no flash comp on the unit (instead you either trust the TTL metering, or set it to high or low power and expose manually). I bought the DG version... I don't mind not having wireless since I will mostly be using it in the worst possible place (on top of the camera)... flash compensation would be nice, but I don't use flash enough to trade the DG in for the Super or for a "first party" flash.
Regarding S2K1's comment, I use mine on a Pentax too and it works fine, so maybe there is a problem with his copy of the camera or flash... maybe I'm reading your post wrong but I'm not sure how your friend plans to use a Canon 430EX on his Pentax
There is one thing about the Sigma that may concern you - build quality. Speaking of the standard DG version... overall it doesn't feel all that bad, but it doesn't slide on the hotshoe as smoothly as a "first party" flash, nor does it have the refinements of a first-party (for example it uses the standard method of fastening the flash to the hotshoe - turning a dial - whereas the Pentax had a much more convenient 'switch')... those are minor things though. The one thing that actually worries me is the battery door - the design of this is terrible and I will be surprised if I'm not holding it on with tape in a year. That may have been improved on the Super; I'm not sure.
Basically if all you want is a powerful flash, and you are happy to either trust your camera's TTL or do it all manually, the EF 500DG is not a bad choice considering how little it costs. The Super has a couple of significant things that make it better, but I'm not sure I'd want to spend more if the build quality (specifically the battery door) is the same as the DG.