NHS web site about Strokes
http://www.nhs.uk/Conditions/Stroke/Pages/Prev ention.aspx?url=Pages/What-is-it.aspx Should give you some info and hopefully point you in the right direction forresources that can help and support the both of you.
Really need to decrease blood pressure, so no smoking, not much drinking, more exercise, healthy eating etc. etc. However, some people are just more prone to these things than others. Now you know there is a problem, you and you Mum can deal with it. Maybe you could do some sort of exercise together, like walking a dog, or a weekend stroll?
Hope it goes well and she is on the mend