Racers are not considered outside assistance.
My understanding was the violation was a forward progress away from equipment, meaning that Heather was making progress away from her bike. If it was a wheel issue and Sym had given Heather one of her wheels and Heather finished on her bike with a different wheel that would not have been outside assistance (racers are part of the race) and would not have been a progress violation.
I have also seen in the USAT rules where this could have been a variable time penalty.
The rule that I heard as being the DQ:
5.2 Forward Progress. Participants shall not make any forward progress unaccompanied by their bicycle. If a bicycle is rendered inoperable, a participant may proceed on the cycling course, running or walking, while pushing or carrying the bicycle, so long as the bicycle is pushed or carried in a manner not to obstruct or impede the progress of other participants. Any violation of this section shall result in disqualification.
Regardless, I would hope that the USAT will clarify the matter to exactly what was the penalty as well as detail a list of scenarios that would have been possible and what the outcomes would have been