Who’s going to strip him of his US citizenship? Jorge Boosh?
A reasonable question.
The Department of State has a complete webpage devoted to this.
This is the main statement:
With respect to loss of nationality, 349(a)(4) of the Immigration and Nationality Act (INA), as amended, is the applicable section of law. Pursuant to 349(a)(4), accepting, serving in, or performing duties of in a foreign government is a potentially expatriating act. In order to come within the Act, the person must either be a national of that country or take an oath of allegiance in connection with the position. Thus, the threshold question is whether the person's actions fall within the scope of this provision. Information used to make this determination may include official confirmation from the foreign government about the person's nationality, and whether an oath of allegiance is required
The rest of the page is somewhat subjective in its content. Basically, it says that individual cases are examined for intent, which is somewhat limp, considering that it allows the dual national to do whatever he wants. But then it does point out that an earlier AG opinion makes the assumption that a dual national taking a position of trust in a foreign government can be assumed to be expatriating himself.
The final decision would appear to be a function of the State Department.
The relevant law is 8 USC 1481.
I would say that unless he has been extremely diligent about maintaining appearances, Sr. Hernandez is surfing right at the edge of expatriation, and might have a hard time proving that he hasn't already gone over the edge.