Normally you would try to get access to the account, but if she has not used it then there may be problems doing that, plust there's probably nothing worth saving. She will therefore likely need to get Apple to reset the Mac completely or she will continue to have problems with the old account being configured on it.
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online* at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide many details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt** issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the first owner who does this, or a descendant with legal proof of ownership transfer). Note that while this will let you get use of the device again, it will not help you with lost access to an account.
* There is a special link for this service. To save time you should use it instead of https://getsupport.apple.com/ .
**Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support
She can also start with a new account. This article explains how to create a new Apple ID (create an Apple Account) --> How to create a new Apple ID - Apple Support
Note that she will need a different email address for the new account.