From what I've read about Texas Method, an option for the Friday is to do dynamic effort sets instead of PR squats. Mark Rippetoe and Lon Kilgore say this (referring to swapping in dynamic effort sets in the Texas Method):
When beginning this type of training, it is normal to continue to use
5 sets of 5 on Monday and replace Friday's workout with dynamic effort
(Dynamic effort sets are explained in more detail in their book.)
The main point is you can do power work (like sprints), and it looks like Friday is the day to do it. My guess is that if you're set on doing Tabata sprints in addition to your heavy squats on Friday, you may have to re-adjust the percentages you're doing across the week to account for the extra load.
As an aside, remember why you've switched to Texas Method, though. It's because the amount of work you needed to do to stimulate adaptation required too much recovery for a novice schedule. The Texas Method gave you a way to do more work (volume day), and have the recovery time you need (light day). Adding in more stuff at this point will be tricky to do.
Rippetoe and Kilgore say this about tweaking the Texas Method:
- If you stall by not setting PRs on Fridays, but are not failing on Mondays, add more volume on Monday
- If you fail to set PRs on Friday and regress on Monday, reduce the volume on Monday (fewer warm-up sets, reduce the percentage, or take out sets or reps from your work sets)
I've given a lot of extra info that you didn't ask about, because I'm assuming that you're wanting to add tabata sprints due to stalling progress. So, I wanted to remind you about the standard tweaks that can be done to Texas Method.