How high is her bp? Obviously, there are varying degrees of hypertension, some that can be alterd through lifestyle changes, others that will need medication such as beta blockers, diuretics or ACE inhibitors.
Is your mother in law overweight? There is a pretty strong correlation between weight and blood pressure.
How physically active is she? I know she's 71, but if she increased the amount of cv work she did (if age related conditions allow)this could also positively affect her blood pressure, especially if the increasein exercise positively affected her weight (if it is high).
The best piece of advice is to listen to the doctors and not us internet nerds! Often nowadays doctors are more receptive to using dietary modification, or exercise, to treat such conditions if possible.
Try to convince her not to worry to much. This can only compound the situation.