Can HLF sued Ackman and his Fund if FTC and SEC clears them out of a pyramid scheme?
They can always sue, but HLF likely does not have a case.
The only plausible claim HLF might have against Ackman would be for defamation, but HLF would need to establish that Ackman's presentation was materially false (ie. its not enough if a couple of details were incorrect, as long as the general gist of Ackman's presentation was correct). Truth is an absolute defense against defamation.
Ackman seems supremely confident that he's correct, betting over a billion dollars on his short position, and as Ackman said if HLF sues him and/or Pershing Square, they get access to HLF's books and records which may well provide even better evidence of the pyramid scheme.