I'd agree with your friend. Ferrer has a GS semi and final, a Masters final, two other wins, two other finals and lots of points elsewhere. He has close to 1000 points more than Murray which I find hard to argue with.
And even if you only consider MS and GS, Ferrer is ahead by around 200 points.
The ATP race is just as deceiving as the official rankings and it's the best system we have for deciding who has had the most successful last x months. I assume you'd also argue that Rafa should be considered no. 4?
Of course, all this will change in 2 weeks when Andy wins Wimbledon!
But only because Murray missed the French Open. Hardly a fair comparison, is it?
If we look at points won compared to tournaments played, Murray is (well) ahead (351 - 291).
If we look at win to loss ratio, Murray is (well) ahead (5.4 - 3.3).
Ferrer is ahead in the race because he's played FIVE more tournaments, including a Slam.