I have had every MacBook Pro in bits. (Apple tech). There is nowhere to put a second CPU. There is not enough physical space for it, there is no room on the heat sink, let alone the logic board. And there is certainly no connectors for it on the board.
The only Macs you could even think of doing this with are single proc G4 or G5 towers. But still it might not work, even if you soldered the sockets on perfectly.
This is utter rubbish.
A 2.5GHz Core 2 Duo would easily outscore a single core, DP G5 by this kind of margin anyway.
Either you are trying to con us, your 'friend' is trying to con you, or one or both of you are idiots. A dual core chip registers as 2 logical CPUs. Or maybe someone was using hyperthreading to fake 4 logical cores. Regardless, its all balls.
This would have to be a custom logic board, and it would drain its battery in no time. Not to mention cook itself. And anyone near it.