Well, I cant really say how much BP meds will effect your mother, it seems very much like some folks find it no problem, others find it a problem.

Anyway, I thought I would give you my own BP experience which so far is around 14Lbs wieght loss in a month (I only wiegh myself once a week) and my BP today is (i only do this usually once a week but for this post...)
my bp a month ago was 160/108 (with the highest reading of the day 175/110 or something like that)
You can check my journal to keep an eye on what my success is.

128/94 p92
129/90 p92
129/95 p90

Now why am I showing you this? Well I have had high blood pressue for years, I was first warned about it in 1994, in a job medical, and then in around 1997 I was placed on medication.

Over this time my wieght has been up down all over the place with various exersize routines ect ect. I have been as low as around 160Lbs, but my BP only ever altered a bit. At no time has my blood pressure ever looked good enough to be off medication. Today, is the first time I can say that!!!

So whats different? well, one thing is a stopped taking my medication about a year ago and decided to do something about it myself (something I would never recommend to anyone, basically it can kill you)
the second thing I have done is cut out the really heavy carb related foods.

My point of this long post, is that no way does losing just 14Lbs give you normal blood pressure, and you might find that even if the wieght loss is slow that the blood pressure drops are fairly fast, this is because high carb foods DO agrevate high blood pressure.

Good luck, tell your mum to stay on the medication but to ask her Dr to give her the lowest dose he thinks is ok as she may have quite a BP drop.