You mention that your partner eats pretty healthy. Virtually all of us who are HIV positive have periods of feeling upset, worried, anxious or depressed. Fatigued individuals should pay close attention to any concurrent symptoms such as fever, cough, headaches etc. Of course, with your partners' high CD4 counts an opportunistic infection would be highly unlikely. So this would be pretty far down on the list of likely possibilities.
Hormonal imbalances can cause significant fatigue.
Decreased testosterone levels are a very common occurrence in HIV positive men and can be associated with fatigue, decreased libido and other symptoms. Other factors could include increased levels of cortisol an adrenal hormone that is produced in abundance during chronic infections as a normal body defense against stress. Low levels of thyroid hormone can also be associated with fatigue. Medications whether HIV related or not can often have fatigue as a potential side effect. Anemia, a decrease in red blood cells is one of the most common causes for fatigue in HIV infection.