Has anybody had experience using a baby monitor for a dementia patient that you want to keep an eye on when you are outside in the garden?

I found a floor alarm mat much more helpful.  With the camera I was constantly checking it, waking up in the middle of the night to see what she was doing.  With the alarm mat as soon as she steps on it (well there is a few second delay) the alarm goes off in my room or wherever I am in the house (the alarm is small so you can move it where you are).  The mat is wireless (batteries only that come with the mat and will last a year then you'll need to order another battery) and the alarm is battery operated (C or D size) or you can buy an plug in adaptor just in case the batteries die or the electricity goes out the batteries will kick in.  The alarm has three sound levels and the loudest one is pretty loud.  I believe I purchased it on Amazon ($178.00).  Be sure the edges of the mat are beveled so she doesn't trip over it and that the alarm sounds in your room and not hers.  Best purchase I've ever made.  You can move the floor mat anywhere.  If she's sitting in her favorite chair just put it in front of it, etc.  Hopefully this helps.

Yes, I used a simple one that had two way communication so I could reassure her if she called me but there are so many options available now. You'll need to be sure to check the range though if your yard is large.