Sounds like what is normally referred to as "bird nesting" or thread nesting which is essentially caused, in the majority of cases, by incorrect threading.
Advise your wife to always raise the presser foot
when threading the machine. This releases the tension disk so the thread will seat properly. If the thread is not seated in the tension disk, the upper thread will loop and knot under the fabric causing "birds nests"--an ugly mess of thread!
Be sure to install new needles frequently. A bent, dull, or nicked needle will be the cause of many frustrations. Also, avoid old or bargain bin thread. And use the proper needle for the thread and fabric of the project.
The correct tension has been achieved when the upper thread and bobbin thread meet in the middle of the fabric. The starting point is usually the midway point between high & low tension number, and then testing and tweaking. This does not mean, however, that the tension needed for corduroy fabric will work on silk. There is usually some tweaking involved with every project, ie needle size & type, thread weight & material, type of fabric (heavy, light, knit, woven, etc.)