Heidi is fair game. She's involved in fund raising aspects of the campaign, including from her work buddies at Goldman Sachs. 60 Minutes had a story a few months ago that said her campaign manager is from Goldman Sachs. Add to that, Heidi is "on leave"; she has not resigned.
Thus, by the rules of the game, Heidi as a campaign staffer is fair game. The other wives are not, having avoided that kind of involvement.
CNN did a report on her in New Hampshire, and it was very positive. I didn't vote for Ted, but there's simply no reason to go after her.
In reality, Ted Cruz's campaign is in it's final stages. The acceptance of that varies, but there is simply no reason at all to make her "fair game."