I think I understand what you're trying to say - but you cannot use the 5th Amendment for a plea if you don't qualify.
"No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury, except in cases arising in the land or naval forces, or in the militia, when in actual service in time of war or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation."
The only way within which the witness can plead the fifth is if doing so would result in the admission of guilt or incrimination of the witness. And if the prosecutor grants the witness immunity, then that issue becomes moot. In this instance, it seems that the witness wasn't involved in the crime, associated with assisting in the commission or furtherance of the crime. One way or the other, it's more than likely that the mother will not have the Fifth Amendment available to use as a plea to avoid the subpoena.