Urgent Help required (Tax )

Hello every one . I need some advice . One of my mate got a letter from HMRC which states that she has underpaid tax some of £2505 for last two years . she told me she had two jobs during that time but both of her employers had the national insurance number and she didn't with held any information to prevent paying less tax. I think Decision for discrepancy claim brought forward by the office is unfair .
I also did not understand the code deviation adjusted by HM Revenue & Customs equated to the amount of £2,505 in unpaid taxes and why was not she informed of the changes earlier ?

Any advice and suggestions will be appreciated

U.ALI