Speaking here as someone who works in and around the DC area and deals with a lot of people who have various levels of security clearances.
Investigators have a pretty good BS detector. Unless he is going to be certified for a very high security clearance such as Top Secret, SCI, or SAP, the investigators will probably not even interview your neighbors. While yes, allegations of impropriety are bad, the investigators do a very thorough job of investigating those allegations, if they determine them to be valid.
If they think that there is some truth to the allegations, they'll ask him point blank and give him the opportunity to explain...then they'll go and verify he's telling the truth
They are very thorough. I'm on a loan with my parents and they have to do a re-certification every 5 years. I have to do a phone interview with the FBI. That's probably not going to happen with your neighbor. Of course you don't want your neighbor to go off spouting things like DRUGS! S3X! ORGIES! all over town, so you'll need to monitor whether or not she is actually targeting you or if she is equal opportunity crazy.