Have your son save the text messages and show them to his lawyer. His lawyer should be able to get it stopped and/or use it as evidence in the custody proceeding. Courts typically frown on such behavior. As far as slander, probably not unless the text messages are also sent to other third parties. As far as harassment, again, your son should take the text messages to his lawyer.
There is no criminal charge for slander against an individual in the State of Texas (though she may be civilly liable if she commits this act). A person commits harassment in Texas if the person, with intent to harass, annoy, alarm, abuse, torment, or embarrass another:
(1) initiates communication by telephone, in writing, or by electronic communication and in the course of the communication makes a comment, request, suggestion, or proposal that is obscene;
(2) threatens, by telephone, in writing, or by electronic communication, in a manner reasonably likely to alarm the person receiving the threat, to inflict bodily injury on the person or to commit a felony against the person, a member of his family or household, or his property;
(3) conveys, in a manner reasonably likely to alarm the person receiving the report, a false report, which is known by the conveyor to be false, that another person has suffered death or serious bodily injury;
(4) causes the telephone of another to ring repeatedly or makes repeated telephone communications anonymously or in a manner reasonably likely to harass, annoy, alarm, abuse, torment, embarrass, or offend another;
(5) makes a telephone call and intentionally fails to hang up or disengage the connection;
(6) knowingly permits a telephone under the person's control to be used by another to commit an offense under this section; or
(7) sends repeated electronic communications in a manner reasonably likely to harass, annoy, alarm, abuse, torment, embarrass, or offend another.
In the middle of custody disputes, people say some pretty terrible things, so whether she has committed this crime would really depend on what this woman said and how she said it. If you believe that a criminal offense has been committed, then call the local non-emergency number for the police, usually 311.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.