Legal Question in Wills and Trusts in Massachusetts

The simple, short, correct answer is "no." Only the person whose will it is can make out a will. Power of attorney or other guardian does not cut it.

However, just because she has a guardian does not necessarily mean that she lacks capacity to make a will. She should meet with a probate attorney to determine if she is able.

No you cannot write her Will. As a guardian, you have a fiduciary obligation to her, including not exercising undue influence.

Moreover, the fact she has a guardian does not mean she lacks the mental capacity to create her own Will. She should meet privately with her own attorney and tell him or her what she wants to do. If she lacks capacity then you cannot write a new Will for her.