IIRC, the reporter said that RT said "She might be in Vegas." We never hear him say it.
As to people dying rather suddenly of heat, even if they are experienced, I'll just leave this here:
Park officials: Grand Canyon death apparently heat related
As a Grand Canyon hiker myself, I've actually seen it. A fit man who was trail running thought he'd "cool down" at Indian Gardens (it was about 95-100 degrees, not even that hot, but he had been running from the bottom) and was talking to the ranger and some other people when he just flopped over. He didn't die, but he came close.
I've experienced it myself and had no clue I was already rising in core temperature (heat exhaustion can cause euphoria - it's actually an underlying reason why people die in the desert; it's beautiful, it's hot, you feel GREAT, and you don't realize that you're about to pass out).
If I hadn't been in the company of someone who knew me well and knew I was acting strangely (and touched my forehead), who knows if I would have noticed.
It was about 105 on the day Barbara went missing (at 2-4 pm, which is the alleged time she was out there, which our VI said LE confirmed to him). If it is true that she was wearing so little, and not carrying water, she might have been both dehydrated and hot.
Sure, she was experienced but as we get older, our bodies can react very differently to what we expect.