just how "illegal" was the old content?
Title is rhetorical, I don't want to know.
Secure erase doesn't completely erase a drive, but only about 99.99%. During normal operation, sometimes drive sectors "go bad". What that means is the drive must employ its data recovery algorithm to retrieve the old data. When that happens the drive will automatically rewrite the recovered data into a new sector so that it can be retrieved more readily in the future.
Secure erase won't erase old data from those "bad" sectors, since they're no longer visible to the operating system. But the data there is either 100% recoverable or at least 99+% recoverable by people with the right expertise.
So, if had something of national security interest to erase, e.g.
the US Army guy recently arrested for leaking classified documents, I'd be worried about every single sector on my laptop. That comes under the category of "don't mess with the Eagle".
Also I wouldn't bet against a determined enough adversary (e.g. "the Eagle") recovering traces of data from an erased sector, even if you overwrite it using secure erase. The heads won't be exactly aligned on the rewrite. So maybe 1% of the old data of some sectors might be recoverable by the right equipment and technique, maybe 50% of the old data of other sectors. Even if you recover just 20% of the data from just a handful of disk sectors, that might be enough to prove that something really bad (e.g. a classified document, or child pornography) was once on the drive. Physical destruction of the disk platters is the only way around that.
But if it's just run-of-the-mill "questionable things" a typical ex-husband would do, then secure erase should be more than adequate, and save the expense of a new hard disk.