The normal test for making or changing a Will or a Trust is whether the Maker of that Trust is of "sound and disposing mind." Generally, if a person suffers from Dimentia they are of neither. You last comment really discloses their present status which is they are on the "rebound." However, that doesn't mean they're over their Dimentia yet in my book.
I suggest you wait until a doctor clears them. Why not ask the doctor if they are of sufficient soundness of mind to be able to change such a document. That way, you and he/she has the necessary backup to justify such a change.
A person must have "capacity" to amend their trust or will. That means both legal capacity - being of sound mind, an adult, not subject to a conseratorship, etc., and testamentary capacity - the ability to understand who they are, who their family is, and to understand their assets.
Assuming that the person who has dementia does not have capacity, the only way for the trust or will to be changed is if the ability to change it has been granted in the documents to another person who *does* have capacity, or by a Court on petition from the person with dementia's Conservator or agent under a Durable Power of Attorney.
If you desire the change, check into having a conservator established for the person with dementai. If you desire that the change not happen, try to establish the state of dementia or, again, have a conservatorship established for the person.
It is possible, but you will need to determine if they currently have the sound mind necessary to execute a will under the laws of your state. See and attorney in your local area. Often times it is prudent to have another attorney do an independent review and document that the trustor has the ability to sign the document.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.
Despite having dementia, a person can sometimes still have the capacity to make a Will or change a trust. That said, such a change is going to be more susceptible to challenge. Therefore, it is important that the circumstances of the meetings and the signing are well-documented. The attorney should meet with the elderly person alone to determine capacity and that the change is what the elderly person wants. Also, even if your state does not require it, it may be helpful to have witnesses present at the time the document is signed, to have the elderly person answer questions relevant to the document s/he is signing, and then have the witnesses write up their observations and the reasons they believe s/he had capacity and understood what s/he was signing. A doctor's statement, as mentioned by another poster, would be very helpful. Lastly, if any family members are affected, you could also consider a family agreement agreeing not to challenge the trust document.