There might be some one on the Framebuilder's forum who could explain front end geometry. Perhaps you and the customer should study up on the subject before investing any money on a new fork.

A straight leg fork can have the same rake offset as a curved fork due to the angle at the crown. If you are saying that the 28 mm rake offset is one of the dimensions and that the new straight leg fork will have the same dimensions then there should be no problem. And I've learned something new, since I've never heard of a 28 mm offset.
If he wants a shorter wheelbase he'll need a new frame, and probably a fork with a different rake offset.

I'm guessing that the 28 printed on the fork steerer is perhaps the steerer diameter?

I've never heard of a 28mm rake figure, but if it's a track frame with a ~77-degree head angle that might well be the right figure.

I've got a 1952 E. Christophe road bike, all original, and the head angle is 77.5 degrees. The fork looks fairly normal, but the rake figue must be significantly lower than 38mm in order to balance out the trail figure with the steep head angle.

Reducing the fork's rake will put the rider's toe in the path of the front tire when the bars are turned during slow cornering, possibly toppling the rider.

It will make the bike longer, which is the opposite of what your customer wants.

Ok thanks guys, He has actually did the math and his headtube is 72 degrees, and he ordered a 38mm rake straight fork from his 50mm oem fork.