The cable bill can prevent her from getting cable in the future if there's only one cable company in town (is that really a problem, though?)
But yes, bad debts disappear from your credit report eventually, and the ability of the creditor to collect on them also disappears -- based on the SOL by state. That doesn't mean that they won't try to collect. Any penny she pays towards that debt reinvigorates the SOL, so even if it would have been "uncollectable" (legally) in a few months, if she pays $0.01 towards it she has to wait the full SOL before it becomes uncollectable.
As for the morality of the situation? It's equivocal, IMO. The company collecting the debt has bought the debt for pennies on the dollar; they know most people don't pay it and are taking a risk in trying to collect it. Like all investments, sometimes those risks bear fruit and sometimes they don't. Same goes for the original creditor -- they took a risk, they lost on that one, but probably gained on another one. It's a corporation, it's doesn't have feelings ;) And if they're not looking for her, I don't think she should go looking for them!