I can't believe they'll be backtracking to DFW on same day as arriving in LAS? I imagine it's a different day.
I can offer mostly different experiences to themax I've found LAS to be a very efficient airport when arriving off the two LHR flights; plenty of desks open and often through and out airport in around 30 mins. Last time was so swift had a surprise Limo arranged, and they weren't even there as the arrivals process was so swift