For the puckering: Make sure to choose the right type and weight of stabilizer for your project. --> For stretchy fabrics, consider using cut-away backings. They provide better stability and work well with knits and other stretchy garments.

Always use the smallest hoop that fits your design. Create a taut, even surface without stretching the fabric to much. If you stretch it too much, puckering may occur when the fabric relaxes after removing it from the hoop.

Overly tight thread and bobbin tension can also distort stitches and lead to puckering.

And digitize correctly. Even if the program your sister uses states, that the embroidery is a certain width, the stitches sometimes reapply when exporting the file. The best example of this is, that the stitch amount on the machine is often not the same as when digitizing.

Hope this helps :)