Owenski

I've gotta be honest pal, as I assume your misses mates are female and thus violence is out of the question but I'd be kicking up a proper s**t storm about this camera, her mates would most certainly be banned from coming near the place until its returned.

Doesnt matter the excuses I'd have zero tollerance for someone:
a, knicking it and
b, others pretending nothings wrong with that

Doesn't matter about anything else, you worked to get the money to buy that camera no one has any excuse for taking it. You're misses is free to see her mates away from the property but until the camera is returned no one who was there that night is allowed back in. Simple.