The landlord, or Realtor, they will take care of it! I know of tenant who skipped out on their rent last year, then, the home they thought they were moving to, fell through, they ended up staying and not paying, did get evicted and lost in court!
If you know the homeowner I would let them know. If you don't and there is a real estate agent or a property management company that manages the property you could give them the facts and let them check it out. The way I think about this is "if this was your investment property - would you want someone to let you know?"
First of all who are you in connection to the renter and landlord? How are you privy to this information? Are you going by hear say? Why would you want to involve yourself in someone else's life or issues? What if the renter had a change of heart and decided to stay? Then you have a renter who home situation is in jepardy because of your hear say information. I agree with Nonrealtor. This really isn't any of your business! This is a matter between the tenant and landlord not a third party.