1.During her weightlifting workout, her muscles generate energy (ATP) primarily through a process called aerobic respiration, where oxygen is used to break down glucose into ATP molecules, providing the necessary energy for muscle contraction. Additionally, muscles could use stored energy in the form of creatine phosphate (CP). Another way that uses energy stored within carbohydrates from the blood or from muscle cell reserves to generate ATP. 

2.During her workout, her nervous system sends signals to her muscle cells through neurons, triggering action potentials that result in muscle contraction. The frequency and intensity of these signals are adjusted by her nervous system to match the demands of the exercise.

However, this contraction isn't usually strong enough to fully shorten the muscle cell and it's more like a partial contraction.

To fully shorten the muscle cell and keep it contracted, it needs to receive multiple signals, one after the other, from the neuron. The more signals it gets in a row, the more the muscle cell shortens, and the longer it stays contracted.

When she workout, the pulling of actin by myosin provides force in her muscle, therefore adding more actin and myosin will get more force. That will also make your muscles thicker and appear more muscle when she continues lifting heavier weights.

   I would explained my friend that during weight lifting, muscle contraction requires energy, and when ATP is broken down

oxygen need to breakdown produce energy in the form ATP (adenosine triphosphate) to contract and perform work. 

 When the muscle cell is at rest, it uses some of its ATP to add a phosphate group to creatine, forming CP. When ATP levels drop during intense exercise, CP steps in. It’s rapidly broken down to generate more ATP, providing a burst of energy for about 8–10 seconds. Motor neurons transmit electrical signals from the brain , spinal cord to the muscle. When lifting weights, the nervous system coordinates the firing of motor neurons, which stimulate muscle fibers to contract.The frequency and intensity of these signals determine the force and duration of muscle contractions.Consistent work out perform training, muscle fibers increase in size due to increased protein synthesis and the addition of contractile proteins actin and myosin. Aerobic exercise promotes mitochondrial growth within muscle cells. More mitochondria mean better ATP production through aerobic respiration.