Is over charging the phone while using headset risky
Well, you say "considering even the rarest of the rarest scenarios" and this opens a big can of worms. I assume you don't want us to give borderline movie-like scenarios (e.g. the girl was sleeping on a water mattress and she had so sharp fingernails that she cut into the mattress spilling the water all over the place, including AC wall socket, etc..).

If we consider only plausible worst case scenarios, the answer may be yes, but it's indeed a very unlikely scenario. If it really happened it is probably due to a failure in the charger that triggered a nasty chain of events. 

The charger must convert AC mains voltage to the smallish ~5V required to power modern phones, therefore any sensible designer should have designed it to have reasonable isolation between the parts connected to mains and the low voltage circuits. Usually the isolation measures must meet some safety standard for the country where the charger is sold. These requirements are not the most strict for these household appliances, because they are not intended to regulate devices that are ultimately connected to human body.

Medical equipment, for example an electrocardiograph, must meet much more stringent regulations, especially if powered from mains, because it is understood that in worst case scenarios there could be a person connected to the device and the risk of electrocution must be minimized in case of failures.

The very existence of these stricter regulations for medical equipment (even for non-professional ones, i.e. those that may be used at home by non-professionals) is a hint that the safety standards needed for common household appliances, such as a phone charger, are not deemed enough to protect human life in worst case scenarios where a person is connected in some way to the device.

Anyway, as I said at the beginning, it is really an unlikely scenario, unless:


the charger was damaged or has been heavily abused, and/or
the charger was a low-quality/non-original "cheap" one which wasn't designed with basic safety rules in mind (in some countries it is illegal to sell such things, but you always find cheap ones with counterfeited safety regulation approval marks).


If you want to see what kind of low-quality chargers are sold, this EEVBlog video of a fake Apple charger disassembling is interesting!

After a bit of googling, here and here are two news articles referring to cases similar to the one you speak about. In both cases the culprit seems to be a faulty/not-compliant charger.