Your friend is working on lower body and has been doing one set of 8 bodyweight squats each day. give specific suggestions on how your friend can apply the principles of progression and overload to her current squat routine to see improvement in lower body strength over time.

The suggestion you should give your friend based on the principles of progression and overload is: "You should gradually increase the burden of your training."

Explanation:

The principles of progression and overload were created based on the story of Milo. In ancient Greece, Milo was known for his superhuman strength. According to legend, Milo began raising a calf every day as a boy. Over time, the calf grew and became a bull, and Milo's strength increased in proportion to the calf's mass increase.

Based on this, these principles state that in order to achieve progressive gains in training, one has to progressively increase the training load (either by increasing volume, increasing weight, or increasing intensity). From this we can conclude that the best advice to give your friend is:  "You should gradually increase the burden of your training."