We get this question all of the time. The main reason is that mattresses are no longer a standard size and most are much thicker now than in the past since the “high profile” mattresses are considered more luxurious. A very common complaint years ago was the fitted sheets were too tight or didn’t have long enough sides to completely cover their mattress. And certainly now long enough to have bit of tuck under to prevent the fitted sheet from riding up the mattress and coming “undone.”
Now because of the wider mattresses, manufacturers are typically making them 15-17” deep, which is fine for some pillow top beds, but way too much for the older mattresses, which used to be a standard 8-9” thick. It’s even too much for some 12” thick mattresses. T here is also a trend to make them oversized and this just adds to the problem.
We have a simple fix. We have our workroom just cut them down, usually after a few washings so we know what the true size is after shrinkage. It’s kind of like getting your pants length altered because it is too long. We are so used to just putting a new sheet on the bed that it seems like a real nuisance to wash it and then return it for altering. But it will fit perfectly. We also make fitted sheets from fabric for odd sized or shaped beds, e.g., a round bed.
white top sheet plain white with three lines of blue embroidery. The pillow case has just one line. Surprising that she would buy a set with the case not matching the sheet. Why not compromise and get 
The guys have more interesting sheets. Mr. Big has a set with 1” appliqué on the face and on the edge of the sham, black on grey, similar to Matouk’s Legato, which has three rows rather than two, sitting in front of plain light grey solid. At least I think they’re light grey. They could also be white, hard to tell. He also has a silk quilt with rows of quilting near the edge, like Kumi Kookoon’s “Classic Silk Throw.”