I bought an ex-riding school horse. He was finding the riding school routine too much and looked really sad and stiff. He was about 17 at the time too.
The minute I bought him he completely changed. He was really happy - he too was completely safe on the roads and on his own in the forest was fine. He did - on a couple of occasions - tank off with me when he wanted to race the others - he also learnt to buck if I tried to hold him back. I think many years of riding school hacks where he had really done what he wanted took its toll and he didn't like being asked to control his canter
Saying that - he really enjoyed his life out of the riding school and his "naughty" episodes were few and far between and the enjoyment I had with him far outweighed the bad.
If you do decide to share this horse, she may test you initially - particularly as she is probably used to hacking out with others and may feel insecure on her own - but if anything like my boy - it will be worth it.
Hope this helps a bit