I was on Spironolactoneand Fincar for almost a year before starting Oestrogen about 6 weeks ago now, and that was one of my main reasons for starting before going back to South Africa. Hormones are important for bone health, so your friend has reason to worry.
That said...
You should be okay for up to two years on just anti-androgens, according to most of the different sources I queried. Of course, this will vary from person to person, but if you start anti-androgens at 16, you should be fine getting onto Oestrogen at 18, all SOC kosher.
The muscle cramps and spasms are fairly common when using Spiro, mostly caused by the imbalance between sodium and potassium created in your body. I'm not sure about how Dutasteride would be causing similar symptoms, but I can imagine that anything inhibiting hormonal action would affect muscle function because of that. I know that, when I was on the anti-androgens, I had episodes of muscle-weakness and tremors. Now that I've started oestrogen as well, those are disappearing fairly quickly.
Good luck hon. Hope it helped.
~Simone.