if that is all u want

here
Amazon.com: Heath Zenith Wireless Entry Alert Chime SL-6168: Home Improvement

the senor and magnet go on the door that comes into the room (completely removable does not break any agreement)

the chime goes in your room lowest setting for sound and put something over top like pillow or blanket folded up to muffle the sound lvl even more anytime the door opens or shuts u will be chimed ........or u can hang in hallwall so both u can hear it
as for your room
Addalock - (1 Piece) - Door Levers - Amazon.com
Door/Window Entry Alarm with Magnetic Sensor, Pack of 2 - Household Alarms And Detectors - Amazon.com

the lock is something that u can take with u when u leave but at night time it gives u atleast10 secs (if they have a shotgun with right rounds) but normally several mins since most ppl do not

the other thing is like the chime but louder and only ment for your room when u are in there put it at the top of the door and have it to chime when it opens or alarm

the rest is up to u since she seem shy .........clear the air with a friendly talk .....get her something she likes muffins cleaning supplies ......ask if she is ok and clear the air get on a good spot and let it go (secretly keep a tazer in your room )