I cannot for the life of me see why not, but she MUST get her application properly signed and witnessed as when people's apperance has changed significantly since the previous issue, they don't take these on trust.
I did an "on the day" renewal last year and it was quite easy but you have to a) make an appointment in advance and b) be sure to remember to write down the reference number they give you over the phone. I forgot to do this and had to phone France from a public call box to get my o/h to look it up for me.
Telling them my name and address etc was not sufficient.