Why does she need her UTR?
If she's trying to submit an online tax return (as many people will be trying to do over the next few days) it's not her UTR that she requires but her Government Gateway ID. If she's not got that it can take 10 days to obtain it through the post (putting her beyond the deadine for submitting her tax return):
https://online.hmrc.gov.uk/shortforms/form/OSHGGIDInd
However the trial online identification service (taking just 10 minutes) might help her to get around that:
https://www.signin.service.gov.uk/start
If she still requires her UTR then it should be on any paperwork from HMRC, alongside 'UTR', 'Tax Reference' or 'Official Use'.
If she's not got a UTR at all, then she needs to register for self-assessment in order to get one.