My guess it is pure speculation on here only. But many of us here delve into theories and speculations. Only the taxing authorities, state and federal, outside of the entity management and maybe an outside CPA, know who signed the tax returns. As long as everything else is on the up and up, those authorities really don't care who signs the return. Tax evasion is usually another charge LE will drop on someone if there is illegal activity going on. I know when a drug task force was organized in our area, one of the local vice cops told me that in addition to local and state LE and DEA, they had an IRS agent on the TF. Even if they didn't get drug charges to stick tax evasion on unreported income was where the IRS came in.
Signing a return and then claiming not to know about illegal activity, even if true, usually doesn't work with the IRS. This happens most commonly with an innocent spouse signing a joint return. One of the most notable cases is the dentist, Dr. Richard Chentnik and his ex-wife, Cathy Lantz. The IRS went after Lantz in the 2003-4 timeframe for unpaid taxes on Chentnik's Medicare fraud scheme in 1999-2000. She tried to claim innocent spouse defense, but the courts kept agreeing with the IRS till the Tax Court ruled in her favor, but that took until 2009. You sign the return, you're on the hook for any ill gotten gains and unpaid taxes, whether you knew about it or not. Sometimes LE might use this as leverage for testimony.
Again, NOT saying Heidi signed ANY tax returns other than her own. For that matter she may never seen a single return, let alone sign one. From the facts we are given so far there is NO way to tell if Heidi even knew about tax returns, let alone sign them. The closest we come to that is that she is named on some filing documents as the 'controller'. If she indeed acted as the controller she likely played a role in assisting a CPA in preparing returns. I've been involved in that from both sides - as a controller and as a CPA. Of course, this is based on IF she was, in fact, acting as controller and was not a controller in title only.