Okay, I’m going to defend Health here. He didn’t come here to bash a particular person, or insult a ger. He came to pose a halachic shaila, based on an observation he made.
Sure, it’s possible for a shaitel to look real; it’s also possible on occassion to know for sure that it’s not a shaitel.
Of course, it’s possible that someone, over time, became lax in her observance; but since there are many shomrei Shabbos who don’t cover their hair, it’s very possible that she never intended to, and it’s certainly reasonable for his experience to arouse the shaila.
However (and I may surprise some with this opinion), I think that even if she never intended to cover her hair, her geirus is still valid. We don’t need to find a legitimate heter to leave the hair uncovered, all we need to do is be able to assume that she doesn’t think it’s a violation of halacha. That’s reasonable to assume, since so many shomrei Shabbos ladies don’t wear a head covering.
Tosofos in Shabbos (68A bd”h Ger – http://hebrewbooks.org/shas.aspx?mesechta=2&daf=68&format=pdf) say that even if someone doesn’t know about Shabbos, it doesn’t passul the geirus.
I was present when this very shaila was presented to an adam gadol; there was a lady who was m’gayer with no intention to cover her hair (she converted with her husband). The godol affirmed the geirus.