Can she leave the UK 19 days after the expiry date?
That is definitely a No No for the U.K.
You have to leave the UK on or before the expiry date of your visa. Or in case you have a long term visa you have to leave within the period given to you when you were admitted to the UK, even if your visa lasts beyond that date.
Is she allowed to travel from Fiji on the 22nd since the visa is not yet valid?
Depends upon the airline. There is no well known hard and fast policy for this.
With this ticket she might be refused entry. Even if they don’t ask her about the return ticket, she will mention the duration on the landing card.
This ticket will need to be changed and while you’re changing it also change the outbound leg for peace of mind.
A very important note by @Zach Lipton:
She should only stay six months in the UK if that's what she said she would do on her visa application. If she said she would stay for a shorter time, she could be refused entry or refused a visa in the future if she suddenly extends her stay to six months when she said she wouldn't be there that long.