[For our US friends, the 'humans rights act' is a piece of nonsensical
European legislation, that  is often used as the imaginary reason (or excuse
when someone doesn't want to do something) why you cannot do numerous
things, some of them no doubt correct, others not, and in each case needing
to be tested by the courts before its determined whether something does or
doesn't offend. People (like this second 'friend') tend to convict
themselves or others and constrain their actions on pain of imaginary
offences without the law actually having to do anything. Very 1984-ish.]

Now, having got that out of the way;
1) You can already buy devices like this so making one is pointless unless
its just your hobby and you fancy doing it, some care homes use them to
sound an alarm when a resident 'escapes', they ahve been discussed here
before.
2) If your friend needs such a device then almost by definition, she has a
bigger problem, it sounds as if her husband is already in a dangerous
position because she cannot, by herself look after him and is looking at
technology to fix the problem (what's she going to do if he removes it, she
falls asleep, etc etc).
3)Some 'prisoners' in the UK are already tagged for house arrest in a
similar manner, you can have devices that will broadcast your position as
you move around the country, so there certainly is nothing in the HRA that
specifically forbids such devices.
4) Don't convict yourself before you did something! What about his 'human
right' to be safely cared for and not wander into danger? Maybe that is
being violated by her caring for him on her own 24x7? See what I mean,
there's no end to the damn thing, you are both guilty in either event. Do
what you think is best and ethical, not what you think 'officialdom' might
care about.
5)Try getting some help from 'officialdom' rather than living in fear of
them, they are meant to help us, not us be their slaves. Why are you needing
to build such a device when officialdom is meant to help get the right care
for people?
6) Check to see if there is some way you can more meaningfully help your
friend. For example, does she get help from social services, her husband in
day care to give her rest, all the right financial benefits, etc? All more
useful than a device that might tell her husband is nowhere to be found when
she wakes up 2 hours after he went off somewhere. Or that doesn't warn her
because the battery failed or it otherwise went wrong so she didn't go
looking for several hours after he left because she assumed he was nearby as
there was no warning.
7) There are a couple of uk legal groups (I'd recommend the moderated one)
if you are still worried about what officialdom will do to you, rather than
what they should be doing *for* you. But with the HRA its all opinion
anyway, there is no way of knowing what the law actually means until you go
to court.

I know that more than one care facilty for Alzheimer's patients uses a
system of bracelets and a perimeter wire that warns them when people are
going out of the building -- sort of an invisible fence for people.  I
don't see how what your friend wants is any different.

Dana

I am from WI, we have a great resource to help with elderly or
handicapped people, it's called the Aging Resource Center. They
provided me with something called 'Safe Return" it is a  bracelet that
is worn by the 'patient' it contains an ID number. If this person is
"found" , they would call the phone number on the bracelet, and give
the ID number on the bracelet, this company would then call the contact
agency or family that is listed.
When I signed my father up, I was told it was nation wide. I actually
got a locking bracelet type, similar to the medic alert bracelets, and
put it on next to his watch band. It was extremely difficult to get
off.
Also for at home, there are alarms that can be placed on doors, that
when activated and the door is opened an alarm will go off.
There must be agencies in your area that would offer advice or
assistance. Call, thats what they  are there for.
Good Luck