Since he's not using the granny his total chain wrap is 14 (53-39) plus 13 (25-12) = 27 so the short-cage derailler would work fine. In fact, since it isn't designed to accommodate a 21-tooth difference in cassette cogs, as it would with an 11-32 cassette, it sweeps across the cassette at a shallower angle than a long-cage derailer, thus keeping the guide pulley closer to the cogs, leading to more precise shifting. The only negative would be a chain line disparity when using the 1 thru 4 cogs with the 53-tooth chain ring. But then, that's what the 39-tooth ring is for! Good to go!

Regards,
Bob P.