First, what she is doing or trying to do to your son is considered to be along the same lines as blackmail.
I read over the Texas Penal Code and saw the section which you are referring to and it does refer to intentional or knowing conduct and his defense to that would be that at the time of this incident he did not have the intent to knowingly or intentionally act due to his incapacity after consuming alcohol/pills.
However, I did read another section of the code which could pose some problems for him which I will post below.
Sec. 21.11. INDECENCY WITH A CHILD. (a) A person commits an offense if, with a child younger than 17 years and not the person's spouse, whether the child is of the same or opposite sex, the person:
(1) engages in sexual contact with the child or causes the child to engage in sexual contact; or
(2) with intent to arouse or gratify the sexual desire of any person:
(A) exposes the person's anus or any part of the person's genitals, knowing the child is present; or
(B) causes the child to expose the child's anus or any part of the child's genitals.
(b) It is an affirmative defense to prosecution under this section that the actor:
(1) was not more than three years older than the victim and of the opposite sex;
(2) did not use duress, force, or a threat against the victim at the time of the offense; and
(3) at the time of the offense:
(A) was not required under Chapter 62, Code of Criminal
Procedure, to register for life as a sex offender; or
(B) was not a person who under Chapter 62 had a reportable conviction or adjudication for an offense under this section.
(c) In this section, "sexual contact" means the following acts, if committed with the intent to arouse or gratify the sexual desire of any person:
(1) any touching by a person, including touching through clothing, of the anus, breast, or any part of the genitals of a child; or
(2) any touching of any part of the body of a child, including touching through clothing, with the anus, breast, or any part of the genitals of a person.
(d) An offense under Subsection (a)(1) is a felony of the second degree
and an offense under Subsection (a)(2) is a felony of the third degree.
There a million questions that need to be answered at this point. What is her age, whether or not they actually had sexual contact, why she has not reported anything to date, etc. It seems as though she is merely trying to threaten him with the possibility of her going to the police. I would tell him to tell the girl that he needs proof of the claims she is making.
Best of luck.