How secure is data overwriting?

A friend of mine sold her Huawei P20 Lite. Your friend had all deleted data overwritten several times with a Diskdigger app. Then the phone was reset and overwritten again.

Allegedly the buyer is a hacker and he said you can restore overwritten data with a forensic Sowtair that the police also use.

How likely is it that overwritten files can be recovered

The probability is almost zero and if someone sounds like that, he is more of a blender than a real expert. Also, the context eludes me, why he even says something like that. With some media, such as SSD, it is not about overwriting the data used, but about everything.

50/50 like everything

It is possible. The only question is is the effort worth it?

Very time-consuming and expensive something like that. Especially if you have already overwritten the memory several times. Almost impossible with electric storage.

So by electronic storage you mean cell phones? So it is impossible to recover overwritten data

So with a cell phone where the data has been overwritten multiple times, is it not possible?

Just because you think it has been overwritten doesn't mean that it is true. Depending on the software and medium, the data is not actually overwritten, but simply stored somewhere else.

99.9% that he bluffs and doesn't make it. Of course it is possible, but the effort is simply too huge and costly.

Unless you had plans for an atomic bomb on your cell phone.

With electronic storage devices such as those used in cell phones, I think it is very presumptuous to assume that data can be restored.

Diskdigger & Co is pretty much the last thing that makes sense with flash media.
That only speeds up wear and tear.

Either your smartphone is capable of securely deleting data or using encrypted data, or you can't rely on everything (!) Being and being securely deleted.