Submitting original docs after application?

[hopingforchange]

Asking for a friend. She is applying from within the UK to extend her ancestry visa, which expires on Tuesday 18 June. She has just realised that she needs to include her mother's birth certificate. Unfortunately the original is in New Zealand and it will be very difficult to get it to the UK in time.

Is it ok to send it in a few days after submitting her applcation? If so, how should she deal with this in her application - e.g. should she explain the situation in the cover letter?

Thanks.