It would be a really good idea for your friend to try and get a referral to the rheumatology department at Great Ormond Street Hospital as they are specialists in juvenile dermatomyositis. In fact they along with UCL house the Juvenile Dermatomyositis Research Centre in the UK. They have juvenile dermatomyositis clinics every fortnight on a thursday.
Please see the link below for how to get a referral:
Here is a factsheet about juvenile dermatomyositis:
With regards to your questions, the sooner your friends daughter starts getting treatment the better, so that muscles do not waste too much. Treatment involves medications (there are a number of options on this front) combined with physiotherapy. Occupational therapy may be needed if she has problems moving limbs and doing things etc. The outlook for JDM is usually good and most children will make a complete recovery eventually but again the earlier the treatment begins the better the outlook - early, aggressive treatment is the way forward.
JDM is treatable with medication aimed at controlling the disease process until it goes into remission - so please dont get too disheartened.
I am not aware of any alternative therapies though I am sure there are some. However it is important that medications are used if they are needed.
One thing to consider is sun exposure. The rash associated with JDM can be made worse by the sun and there are other effects known too. It is a good idea to apply high factor 30 suncream when the girl is going outside and wearing a cap/hat would be a good idea too. Also during the middle of the day when the sun is more intense to keep in the shade or inside.
I really would recommend trying to get to Great Ormond Street Hospital though, they really are amazing at treating this condition amongst other rheumatological problems.
I hope this is of help and wish your friend and her little angel daughter good luck for the future. Please all keep positive!