I'm not a climber, and I haven't visited
Hueco Tanks State Historical Park yet, but I can tell you your friends are right... West Texas, especially El Paso (where Hueco Tanks SHP is located), can get sizzlin' hot in the summertime.
Put it this way... we had temperatures in the low 90's during the day
in September when we camped in
Big Bend National Park. Thankfully, there's some signficiant elevation variation in Big Bend so when we were in the Basin area (mountains), for example, it was not quite so hot as when we were down by the Rio Grande River -- which happened to be where we were camped (Rio Grande Village campground).
So, yeah, I'd reserve West Texas desert type adventures for times other than summer... at least if you're a Texas heat wimp, like I am (just as
Justin, my hubby -- I can't seem to handle temperature extremes quite as well as most normal human beings.)