Clinebarger will have your best answer.
My 2001 Silverado was sluggish between R, N, and D since I bought it new. I almost immediately got a Hypertech programmer and used the shift firmness setting to make it more responsive. In your friend's case, the lag is probably aggravated by age and wear, though.
My previous vehicle was a 1987 Caprice ex-sheriff's car that had a second act with a crop insurance adjuster. I can't remember how it shifted originally, but I bought it at 150,000 miles and immediately put in a B&M Transpak to firm up the 700R4. I think Transgo now has kits that are more comprehensive, and ones that target problems with the original design.