Will taking calcium and vitamin k supplements cure osteoporosis and prevent fractures in an otherwise healthy 86 year old lady?

No. It will neither cure nor prevent fractures. Protective hip covers, fall precautions may help. Use of phosphates will be individualized at 86.
Most likely not. At 86, an elderly lady is prone to osteoporotic changes in her bones due to hormonal changes that come with age. Calcium, Vit K as well as Vit D supplement are being used by millions of people but they are rarely curative but more preventative against serious complications.
Vit D and calcium. Vitamin D and calcium may help in reducing falls and could help in preventing fractures.