Should someone giving a second opinion on an article be a co-author or just acknowledged?
To just evaluate a manuscript should not result in co-authorship. Authorship implies a certain contribution to the article. There are many interpretations of what is sufficient and some are really bad. Criteria for co-authorship that has gained support has been given by the Vancouver Protocol (see e.g. the BMJ description of their use of the terms in practise)


Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work;

AND

Drafting the work or revising it critically for important intellectual content;

AND

Final approval of the version to be published;

AND

Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.


An advisor for a master's project sounds as a reasonable candidate for co-authorship since that person likely has provided much input on original ideas through to the final written product. How that applies in your case is for you to decide.
It is, however, never wrong to add a new person as co-author, if that person provides contributions that qualify them according to the list above. So consider who may be considered co-author and do not promise co-authorship without explaining the terms.
For additional input, please search on the authorship here on Academia.sx and look into the concept of contributorship, a concept more clearly related to contributions than what the term authorship implies, by also searching the internet.