[@Mr Allen J] No, he hasn't "broken" any of the rules to allow her to.  

She can, however, infer that someone is that there other people can see.

His power gets weird, because he has an effect on the environment, but there's a certain level of removal from his direct effect before you are able to process it.  

For instance, if he opened a door and came in, no one would notice that the door had opened.  They could, however, notice a sudden draft/change in temperature/light quality.

Also interesting, because he is not technically invisible, he technically casts a shadow.  If you can't "see" him, you can't "see" the shadow either, but there are situations where you would notice that light isn't behaving properly.  A basic example is if you're lying on the beach and he stands over you.  You'll notice that you can't see the sun anymore, but you can't figure out why.  

It's a super weird power, and it messes with people's very consciousness.  


EDIT:  Actually, I think that if your character can reasonably infer that someone is there, and they can figure out exactly [i]where[/i] "there" is, they can look directly at him and break the filter.

That will be a new rule for his power, but I will be watching carefully to rule on what is a reasonable inference.