As you state the change in carer, her nervousness and resulting lack of attention could all have combined to bringing it on. These birds seem to be attuned to human emotion and it is important to stay calm with them, in spite of this behaviour, as a negative reaction before the bird, may exacerbate the problem. However, it's always sensible to get your bird checked by a professional, to exclude health issues that may be causing the behaviour change.
After excluding health problems, there is a few things that can be done, all the advice stems around health, diet and environment:
Cage – is their cage large enough? Is it clean? Hygienic animals by
nature, a dirty, unkempt cage is detrimental to a Grey's mental as
well as physical health.
Baths / Showers – your Grey should be getting
regular baths or showers to keep them clean and healthy. Also expose
them to moist air – put them in the bathroom to enjoy the steam while
you're taking a shower. No one likes to be itchy and dirty, least of
all an African Grey.
Fragrances - Room fresheners, candles, perfumes,
hair sprays etc, can irritate an African Grey’s skin
Give your African Grey self-confidence – encourage it to explore and be curious. Expose it to change – take it into different rooms in the house, show it different colors and toys, socialize with other people; so it learns change is okay and these things shouldn't make it afraid. A happy bird is a healthy bird (1)
African Grey Feather Plucking / Feather Picking african-grey-parrot.com (1)
The following article also confirms this type of solution and adds some more tips about diet and feeding. Contrary to the title, they are acknowledging that African Grey Parrots can pluck, but don't like them being referred to as "pluckers".
They suggest checking for seed and peanut allergies and reactions. Some seeds can also carry toxins, depending where they are sourced from. These things can lead to feather plucking. The article goes into quite a bit of detail about diet.
- Provide a widely varied soft food diet that is comprised of birdie bread, beans/grains/pasta/veggies, sprouts, fresh fruits and veggies. Choose fruits and veggies high in Vitamin A for their immune system benefits. A plucking bird is usually a stressed bird and may need the immune system boost that high Vitamin A foods provide. (2)
THE AFRICAN GREY MYTHS African Greys Are Pluckers by Bobbi Brinker (2)