Typically, you're looking at about 15 years after
androgen suppression and sexual reassignment surgery to really start to see significant changes in bone density. It's been too early for her to see much of a decrease in bone mass or to make her equal to that of a female. She started off with a much higher bone density than other women her same age, and therefore will maintain a lot of that for a while. Additionally, because she is taking estrogen, that will actually help to maintain that bone mass. Women also have lighter, child bearing hips because of the difference in hormones during the body's developmental years. Her skeleton and body mass and shape developed a long time ago. Those changes cannot be undone. They are permanent.
Her testosterone levels are more than likely in the normal female range, since her adrenals are the primary source for it now. She didn't undergo hormone therapy and surgery until she was fully developed, as compared to someone who completes therapy and surgery in their adolescence or very early adulthood, when they haven't completely developed. She has the potential to be significantly stronger because her muscle development reached several years beyond full maturity, giving her the potential to be significantly stronger than other age matched women.There's not really a way to determine how much her muscle mass will decrease over time. What can be said is that she has a naturally higher propensity to build and maintain muscle mass because she was once a fully developed, adult male. You can't ever take that away from her.