I would tell her pal, god forbid anything happened with you away, you would never get over the mental fact that you could have warned her to be vigilant..

On the other issue, i personally would wait up with a couple of pals, as soon as you hear any noises take a pic with a camera with a damn good flash, that will:

A) Get a pic in case you dont catch em.
B) Blind that twats so they struggle to escape without tripping up.

Then give chase and give them a real kicking they wont want repeated..