As Ruir said, 'who says she works'. The IRS really doesn't care as long as it's books balance, and it works on an annual basis. They could get last year's information fairly easily, but that's not the address they're looking for, now is it?
I think you have too much confidence in the abilities of skip-tracers. Plenty are found, yes, but plenty also fall through the cracks. Consider federal fugitives who often aren't found for years. Most are caught quickly, yes, but some manage to hide.
The easiest way to do what she's doing is to simply shack up with a boy-toy and not have your name on any bills. No credit cards that aren't still at the old address, same with the bank account.
For example, from your link on skiptracing:
Often, the job becomes more than mere research since one must often employ methods of social engineering, which involves calling or visiting former neighbors, or other known contacts to ask about the subject, sometimes under false or misleading pretenses.
If she's only communicating with family through facebook and actually has the geolocation features turned off... It'd take a warrant to facebook to get them to give them IP address/location information.
Records that "skiptracers" use may include phone number databases, credit reports (including information provided on a loan application, credit card application, and in other debt collector databases), job application information, criminal background checks, utility bills (electricity, gas, water, sewage, phone, Internet, and cable), social security, disability, and public tax information
Thus my 'not working'(at least above the table, legally), which takes care of job apps, background checks, social security, and such, 'using the old address for bank/credit cards(doable with the internet), and 'living with boy toy' which takes care of the rest.