(As always, I reserve the right to change my opinion upon learning more of the facts.)
In order for this to be a 'mutual mistake of fact' both parties must have had the mistaken belief that they had agreed to something in the lease which in actuality is not in accord with the plain language of the lease. This is not the case.
The question arises as to what type of 'mistake' this really is. A mistake of fact in contract law is of the type described above: a belief by one or both parties that they were agreeing to something different than what the plain language of the lease shows.
Then there is the other type of mistake which has no legal basis for contract rescission: where one party signs a contract and afterwards professes that he had 'intended' to negotiate something differently, or that he forgot to change something even though he meant to. The only time this can have legal basis is if the party arguing it was 'non compos mentis' at the time of signing.
The notice of the rent increase only presents the landlord's 'intent' to raise the rent. Moreover, it is superseded by the execution of the written contract. Now, the lessee didn't state whether the notice was verbal or written. If it is verbal, then it would also fall under what's known as the "parol evidence rule" (see below).
If this argument went to court, the landlord's attorney's only option would be to argue what's called a 'unilateral mistake.'
A unilateral mistake, in contrast to a mutual mistake, is a mistake where only one party has the mistaken belief.
Architects & Contractors Estimating Service, Inc. v. Smith (164 Cal. App. 3d 1001; 1985):
"A mistake of fact[...] consists of: "1. An unconscious ignorance or forgetfulness of a fact past or present, material to the contract; or, 2. Belief in the present existence of a thing material to the contract, which does not exist, or in the past existence of such a thing, which has not existed." (§ 1577.) A mistake need not be mutual. Unilateral mistake is ground for relief where the mistake is due to the fault of the other party or the other party knows or has reason to know of the mistake."
If the landlord wanted to argue that he believed in good faith that the lease stated something it did not, he would have a very difficult time. Keep in mind that most of the people who want out of a contract try this, and very few succeed.
The landlord would have to:
1) Convince the court that he not only intended to change the rental rate, but in actuality thought he really did change the numbers on the written lease. (Fat chance!)
2) As I noted above, if the notice given for the rent increase was parol, it would fall under the "parol evidence rule". The parol evidence rule states that if there is a signed contract that the terms of the contract cannot be altered by any evidence of previous oral notices/intent/agreement which purport to change, explain, or contradict the contract. Such evidence to contradict the terms of the lease contract would not even be admissable.
3) As I mentioned in a previous posting, experienced businessmen and management companies are held to high standards when it comes to negotiating contracts. For example, I once was in the courtroom when a tenant sued his landlord for failing to pay for cable tv. The lease read "lessor will pay for cable". The landlord argued it was a mistake. It probably was, and the landlord produced published advertisements which showed which of their complexes they paid cable for and which they did not. The tenant did not live in one of the complexes where tenants received free cable. The court decided in favour of the tenant, reason being that management companies are held to higher standards.
4) Cal.Civ.Code § 1625:
The execution of a contract in writing, whether the law requires it to be written or not, supersedes all the negotiations or stipulations concerning its matter which preceded or accompanied the execution of the instrument.