Failing to report a crime you witness is generally not a crime (though there are limited exceptions to that). However, failing to report the crime along with any act of concealment of the crime may be an offense. In this case, for example, if the landlord were to contact the OP and his answers to the landlord's questions effectively help conceal the fraud (which does not mean active participation in the fraud) that may be criminal. In that regard, silence or misdirection when asked certain questions may amount to concealment. If contacted by the landlord he'd be better off telling the landlord the truth rather than responding in a way that helps conceal what the girlfriend has done.

