Obviously load bearing exercise is required for bone health and Garrets suggestion that you get her a weight vest is a good one but a more practical proposal for someone who enjoys going for a daily walk would be to get them into the habit of carrying a small backpack everywhere. This would be a lot cheaper than a weight vest and once she gets into the habit of carrying it she'll soon find herself lugging home fairly heavy loads of groceries, books or whatever without thinking of it as exercise.
As for walking with hand weights I know a lot of people do it but they can be more trouble than they're worth because they're rarely heavy enough to load the spine but they do tend to screw up your gait which increases your risk of developing problems with your knees, hips and lumbar spine.
When it comes to diets for osteoporosis the book to read is Understanding, Preventing and Overcoming Osteoporosis by Professor Jane Plant. It explains exactly how and why osteoporosis develops and how what you eat affects your bones plus there's a handy chart of the PRAL value of a wide variety of foods which should help with changing your diet to one that promotes bone health.