The lease is valid, there is nothing you can do to void it.
BUT the rent is yours (you also bought the lease with the house), and you can sue your "friend" to get it. You can also sue your "friend" for all other costs and troubles.
Your only way to get the tenant out is if he agrees to get his money back PLUS a fee for his troubles (but you can't force him), don't blame the tenant. Again: you can sue your "friend" for all the costs of paying the tenant to move out.
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By the way, your "friend" can't remove the tenant; the tenat have a legal lease; if your "friend" asked for MORE MONEY it's just another scam.
EDIT
It's also false that you can't enter into any agreement with the tenant; you own the house and the lease. Your old friend landlord-tenant relationship with the tenant is OVER from the moment he signed over the deed. Obviously this person is a con-man or a very ignorant person.
Call police is he trespass, and get a restraining order to keep him away from your property (the house, the tenant, and the lease are "YOURS" now). Remember: it's "your" tentant not "his" tenant any more.
The old landlord have only right to "old rent" debts; BUT he has NO right to present and future rent.
THE LEASE AND THE RENT ARE YOURS, so don't forget that.
edit
Since obviously you don't know much about this legal matters, get a lawyer expert in tenant's laws and civil lawsuits as soon as possible.