Any type of walking will have the pizoelectric effect, in fact sitting in an improper posture will also have that affect due to the body fighting gravity.
If I am reading this right what you are asking is in reference to Wolf’s Law where bone density is directly proportional to the stress placed on it. The addition of a couple of pounds will have no bearing on her goals, but jogging/running will have a better effect as the forces of impact are much greater than that of walking with weights, better yet sprinting will increase the stress place on the skelatal system even more(of course these may be contraindicated).
If the need to add external load during locamotion is absolutely set in her mind then an adjustable weight vest would be your best be. With that being said, plain ‘ol resistance training should provide the necessary stress for bone remodeling.