Being placed at a treatment facility does not change the underlying sentence calculation. The East Texas Treatment Facility operates under the same TDCJ rules and Texas law that govern every other state correctional institution, and the standard 85% requirement applies regardless of the type of facility.
That means your person will serve about 85% of their total sentence before becoming eligible for release, assuming they maintain a clean disciplinary record throughout their time at the treatment facility. The nature of the placement as a treatment-focused environment does not create a separate or accelerated release track on its own.
The one thing that can change the calculation is what is written in the Judgment and Commitment Order signed by the sentencing judge. If that document includes a parole provision, then parole eligibility exists and the first hearing could come before the 85% mark is reached. Whether parole is in the commitment order is the most important thing to confirm, and an attorney or the facility's case manager can pull that document and give you a definitive answer.
What the treatment placement does offer is programming that strengthens the case for parole if it is available. Completing a structured substance abuse or behavioral treatment program demonstrates exactly the kind of rehabilitation and accountability that parole boards respond to. If parole is on the table, finishing the treatment program with a strong record is one of the most compelling arguments that can be made at a hearing.
Encourage full engagement with whatever the facility's program requires. That investment pays off at the parole hearing and in life after release.