Where is this property? State laws are different and it would it much more difficult, sometime illegal to answer your questions without knowing this information.
My advice: constitutional privacy issue aside, get an attorney to evict the tenant immediately. If not, your friend could face severe punishment. California for example, although the law is murky on a landlord's liability when a tenant commits a criminal act, some courts treat such cases analogous to dangerous propensity of dogs cases. That is, a landlord has a responsility to maintain the rental property in a safe and habitable condition. When the landlord knows or should have known the criminal acts of a tenant, which endanger other tenants, the landlord will be liable for all the injuries and damages therefrom. In the worst case, the negligence of the criminal act could result in the forfeiture of the property, in addition to the other liabilities. Now the other tenants have already told your friends they are concerned about their own safety, your friend is deemed to possess the actual knowledge and he or she must do something to prevent the criminal act or face consequences.
California law allows a 3 day quit or evict notice to be served on the tenant. This should be the starting point. Again, your friend's state law may vary and he or she should consult an attorney immediately.
