Ok - you are right in a very specific case. If he has an Early 2009 MBP he will have an Expresscard/34. Mid 2009 does not have that for the 15" model. So, I suppose I could have given a more thorough answer - however you didn't address this either and added "like I have" as a caveat.
And if you look at what the original question was, it was "install a second GPU into it or even simply replace it". To that the answer is no, 99% of the time when these type of questions are people wanting to swap graphics cards like you can in desktops, very few laptops have that ability and the people that purchase those laptops are purchasing for that ability.
So the bottom line is, with the information we were given and some assumptions based on what people are really getting at when they ask this question, the answer is no. And those people aren't willing to spend several hundred dollars to do an external (not really portable anymore) to get marginally improved graphics performance when they could just build a cheap desktop with far greater gaming capabilities for that money.
Furthermore, I don't believe there are any (and Mac compatibility may limit this even more) that would give you greater graphics performance than what is already in there. You'd need to buy something like a ViDock and add a PCIe card to even approach/improve graphics performance, again with perhaps compatibility issues.