Not sure what the landlord could do if you told him.
He might be able to enter the property, but unsure what the regulations/contract are behind giving notice, etc. Unless she isn't paying rent or something along those lines, his hands might be tied. He cannot evict her or probably do much without some cause, and a neighbors statement doesn't hold much weight.
On the flip side, if he knows there is brewing trouble, he can be more proactive and not let anything else 'slide' so to speak.
I think you just need to keep your eyes/ears peeled, and contact the appropriate party if necessary.