I'd think that if the lease signing is a recent event and they were diagnosed - officially diagnosed by a physician - as having dementia then you could argue that they were not technically competent and therefore incapable of signing a legal document.
If you want support or information about the laws in your state surrounding this matter, contact your local area agency on aging to seek out some advice or an attorney that might help you break the lease.
If you want support or information about the laws in your state surrounding this matter, contact your local area agency on aging to seek out some advice or an attorney that might help you break the lease.