Recently in Argentina a prosecutor suggested that even if she doesnt like a particular law (which might be unconstitutional) she has no choice because she has to apply it. is this so?
Yes and no.
Unlike prosecutors in countries like the United States, but like other civil law countries, prosecutors in Argentina have a legal duty to prosecute all crimes, and in general, do not have the authority to unilaterally determine that a law is unconstitutional (ordinary courts in Argentina also lack the authority to judicially review the constitutionality of statutes).
This said, there is a mechanism in Argentina to have a statute declared unconstitutional, which the prosecutor could avail herself of in the Supreme Court of Argentina (which doubles as the Constitutional Court for Argentina).
Also, the reality is that it is impossible with limited resources to prosecute every single crime. So, every prosecutor has to set priorities on how the limited resources of the office will be used. Doubts about the constitutionality and wisdom of a law, while not justifications for having a policy of never enforcing it, are legitimate reasons to make those cases a lower priority than other cases involving different crimes in the same office.