This would be trivial to do with the Raspberry Pi and Perl. Most motion sensors are single-wire (well, V+, V- and feed), so just a single GPIO pin. Here's a quick example with a mix of real and pseudo:

use warnings;
use strict;

use RPi::WiringPi;

my $pi = RPi::WiringPi->new;

my $pin = $pi->pin(27); # GPIO pin 27

my $count;

while (1){

    # check if time >= 0900 here
    # ...if not, sleep and next

    my $motion = 0;

    for (0..10){
        if ($pin->read == 1){
            $motion = 1;
            last;
        }
        sleep 5;
    }
    
    $count++ if ! $motion;

    if ($count >= 3){ # 0915 hrs and no movement
        # send email, sound an alarm etc
        ...
    }

    sleep 300;
}
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If i were going in this direction id give her an actual button, im thinking of something like a big staples-easy button. Maybe with an led to remind her it is check in time. A positive action rather than a lack of action.