Pentax may be a limited system based on your friends needs. However, it's your needs you need to consider and whether or not Pentax is or isn't meeting them now and whether or not they continue to produce the gear you will need in the future.
I'd hardly call Nikon a limited system, but your friend moved from there to Canon, so there was (in all likelyhood) something specific he needed that Nikon didn't provide at the time.
There have been people who shot with Pentax who have now moved on because they required something specific that Pentax didn't offer. And likewise, there have been people who shot other brands move over to Pentax for the exact same reason.
"Limited" is defined by the individual.....nothing else.