In the UK trade plates are only to cover a car being driven without tax. Nothing to do with Insurance so sounds like a misunderstanding between you and the courier company.

Indeed. As well as that (in the UK, at least), I don't believe trade plates can be used for the purpose you describe.

Can 2 people be charged for the same offence,...

Yes they can. It happens frequently when a car is loaned by one person to another when the owner does not check that insurance is in place.

...can she be said to have allowed the driver to drive her truck without insurance?

Certainly sound like it. As above, trade plates have nothing to do with insurance. When allowing somebody to drive a vehicle, both the owner and the driver must be absolutely certain that insurance is in place. There is a statutory defence for people who drive a vehicle belonging to their employer but that obviously does not apply here. It sounds as though your friend made no checks. Of course, it would be for the court to decide the matter, but arguing that she was misled because the driver used trade plates is unlikely to fly - especially as trade plates do not provide insurance cover.

The cost of failure in court will be high. She will pay an income related fine of one and a half weeks' net income. She will also pay a "Victim Surcharge" of 40% of the fine and a contribution towards prosecution costs which, for conviction following a trial, will be at least £620. As well as that she will have six points imposed on her driving record.

All of the above assumes this took place in the UK and the sentencing guidance I have provided applies to England & Wales only (though a similar outcome can be expected in the other parts of the UK).