All the good advice has been covered. Although biking (or swimming) is great aerobic exercise it takes weight bearing exercise to help out with bone density. (one reason paradoxically why being overweight helps - hip fractures tend to be in thin post menopausal women, not overweight ones!)
A bone that is supporting weight gets strong and stays strong! I suppose for a biker one could encourage biking standing up more, or even remove the seat on a DF, but that wouldn't be very practical!
Estogen used to be used but now is not now recommended because of a tiny increased risk of breast cancer and possibly heart disease. I don't know if the trade off for someone with severe osteoporosis unresponsive with other therapy would be justified or not!?! Haven't kept up with the current literature or recommendations.