Tenant moved in a roommate

[Joe Ventresco]

I have a tenant that has been with me for a year now.  When I rented she was on her own. After several months of her living there  I start to see that she has a guy staying with her.  It's a two family and recently the lower level became available so I have been there a lot getting it ready, showing and I see the two of them together all the time. I haven't seen her by herself in months. So I ask "is he living with you now?", her response was no on two separate occasions.  My lease says no overnight guest for more than 24 hours without notifying... and so on. So I wonder would others let this go?  Pursue it further? Post a three day?

Looking for some opinions. Thanks!