W&H can avoid legal problems by reporting the suspected crime to the relevant tax authorities (IRS, Pennsylvania and the other state, let us say Ohio). This can even be done anonymously, although doing this non-anonomously provides strong evidence that W&H are not parties to the crime. Filing with a false address could constitute a violation of 26 USC 7206, when one
Willfully makes and subscribes any return, statement, or other
document, which contains or is verified by a written declaration that
it is made under the penalties of perjury, and which he does not
believe to be true and correct as to every material matter
"Could" comes from two issues. First, L has to not believe that this is their true address, and second, the address has to be a material matter. The tax form and instructions require you to supply an address, but do not define any rules for what can be "an address". The IRS does define a concept of "tax home" which is the city where you work, bit that is relevant only for deductions related to travel. At least w.r.t. federal taxes, it is not obvious that this is a material matter, unless one is relying on state-specific standard sales tax deductions as part of deduction itemization. The government would also have to prove that they did not believe that the PA address is their "home address", a position that is facilitated by the lack of indication of what qualifies as a "home address".
Para 2 of the tax fraud statute might apply to W&H, which target one who
Willfully aids or assists in, or procures, counsels, or advises the
preparation or presentation under, or in connection with any matter
arising under, the internal revenue laws, of a return, affidavit,
claim, or other document, which is fraudulent or is false as to any
material matter, whether or not such falsity or fraud is with the
knowledge or consent of the person authorized or required to present
such return, affidavit, claim, or document
As described, you have not taken an illegal action, e.g. you have not encouraged, condoned, written in false information, etc. What you have done is not reported a possible tax crime that you are aware of. There are various federal laws whereby a person in a "special" position is obligated to report a suspicion of a crime (law enforcement w.r.t. child sexual exploitation, etc), but there is no general legal obligation with criminal penalties whereby you must report a suspected federal crime. This is a good thing, because a criminal conviction can be a conditional bar to citizenship, under the category of Crimes Against the Authority of the Government. Actually lying about L's residence moves this into the area of a well-defined felony offense, 18 USC 1001 (lying in a federal matter), which would clearly cause citizenship problems.
The same set of questions and answers would arise w.r.t. PA and (presumptively) OH tax laws. In this case, though, there is most likely a crime – tax evasion in OH. I say most likely because I don't know if they filed a fully truthful Ohio tax form, or a fully truthful PA tax form. But if they lived in Ohio and yet did not report their Ohio income (and instead filed a PA income tax form), they did not comply with ORC Ch. 5747, esp §08 and §15, which incurs penalties (but is not a crime). As with the matter of federal income taxes, W&H do not have a legal obligation to report suspected crimes, but they must not actively aid in the commission of a crime.