It is what is mentioned on the "Single Justice Procedure Notice" which determines the offence(s) she has to answer. The Notice of Intended Prosecution (NIP) will obviously only include the speeding allegation because at that point that was the only offence that the police suspected had occurred. The "Fail to Provide Driver's Details" offence is not committed until 28 days after the request was served and no NIP is required for that offence.
As I said earlier, your wife's case is not the same as somebody who either did not receive or did not respond to a s172 request. In those circumstances prosecutors will usually agree to accept a guilty plea to speeding on the understanding that the s172 charge is dropped. It is a tried and tested strategy which is recommended frequently on this forum (and which is invariably successful). Your wife, however, has already stated categorically that she could not establish who was driving.
Of course, as Rookie suggests, her "diligence" in trying to identify the driver need not end because she has been charged with failing to do so. She could approach the prosecutor at her Case Management hearing and say that her diligence has been rewarded and she can now accept that she was driving. What she should emphasise is that she will only plead guilty to speeding providing the s172 charge is dropped. This is important. At present she cannot be convicted of speeding as there is no evidence she was driving. If she pleads guilty to it she will receive three points for that and, since she has little in the way of a defence to the s172 charge, probably six for that in addition. Whether this would be acceptable to a prosecutor in your wife's circumstances, especially as Rookie mentions, as she has not been "dual charged", I don't know. But if she doesn't ask she won't get.
By the way, your registration number is visible in the NIP you posted up.
