How about extending the wiring so that the alarm (depending on the type) can be
heard from all rooms - or a much louder alarm?

-
Mod as a hooter!

If you dont make some changes, like a two way lock for that door your
dad will die of heart failure before your mother dies from alz related
problems.  I have been living thru that with my alz mother for the
past two years.  I have double cylinder dead bolts on all exterior
doors.  On the front door, I also have an "institutional type" door
knob that requires a key to get in or out, just like my dead bolts,
but it also locks automatically every time the door is closed.  It has
a separate key.  The neighbor has a copy of that key.  When we have
occasionally left my mother alone locked in the house for more than an
hour or so, we have left the dead bolt on that door unlocked and only
that door knob locked.  We have arranged with the neighbor on several
occasions to come over, let herself in and check up on my mother.  It
has worked very well.  I would also recommend keyed locks on interior
doors so you dont have to watch the alz person all the time (ie
watching them in the kitchen, etc.).  If necessary, buy some prehinged
preframed doors for $34.95 at the hardware store, build a simple
unfinished wall to block off the kitchen, and keep the alz person
locked out.  Its a little like a prison but so is the dementia ward at
the nursing home.  My mothers second husband died of heart failure two
years ago because of his exhaustion from trying to watch and supervise
my mother 24 hours a day with no breaks.  Your father will too if he
doesnt try to make things easier for himself.  You run a slight risk
re fire safety and accusations of neglect/abuse, but what else can you
do?  My mother will check in to the nursing home within the next few
days.

Check at HITEC.com or other hearing-impaired supply place for flashing
lights instead of a sounding alarm.

I would put a GPS locator on her.

Good Luck