I certainly agree that there is no need that mom give consent to a "change in the lease".
However, if it is done without mom's consent, mom is no longer obligated on the lease. And I don't think that the lessor being aware of the consequences would permit such an alteration in the terms of the lease.
It seems clear that the mother assumed joint responsibility under the lease not as a tenant in fact, but as an accommodation to her daughter. The consequences being that she has underwritten/guaranteed as surety her daughter's performance of the lease.
And because substance prevails over form, this three party relationship (mom, daughter and lessor) is governed by the laws of suretyship. And it is a well settled maxim of the laws of suretyship that any material variance or alteration in the terms of the contract made between the principal obligor (daughter) and the creditor (lessor) without the surety's consent, discharges the surety.
Um, I enjoy your eloquence but I think it’s simpler than that.
Under contract law there cannot be unilateral changes to the contract. An attempt to make such changes doesn’t nullify the original contract (which your dependency on surety does) but renders the amendment void.
I can’t see a court ever accepting your argument over well established contract law.
Besides, even if you make a surety argument, the implied contract of surety has not been altered. Mom would still only be liable for what she agreed to and the daughter is liable for the same as she was. There are no changes affecting the surety issue therefore it remains intact. Even if the lease is changed as suggested, the contract concerning the surety has not.
And of course your entire premise is based on an assumption that hasn’t even been hinted at as valid. The fact mom also lives there pretty much defeats your argument since mom is independently liable for the lease and as such, your surety argument goes out the window.
Your argument would be so much more convincing if mom didn’t also live there but mom also enjoys the benefit of the lease thereby removing her from being nothing more than a guarantor