Is the lease invalid?

[Nat C.]

Im wondering if anyone can give advice or ideas on this matter.

My friend put a couple in his property and signed a lease with them stating they would pay $1000 rent and $1000 deposit. They turned up to move in with all their stuff but had no cash and said they would pay the next day. he foolishly gave them the keys to move in.  Of course the couple never paid and now wont leave.

I said the contract is null and void as there was no consideration. If it was me, I would remove their stuff and change the locks for squatting in my property with an invalid contract.

My friend thinks he has to hire a lawyer to go through a formal 2 month eviction process.  

If I signed a contract to buy a property and the seller signed a contract to sell a property for $100,000 but I never paid the money, that contract is null and void and I have no rights to the house.

What say y'all?