User blog comment:IamJakuhoRaikoben/Chapter 402: Iron Fist of the Fire Dragon, Review./@comment-1359322-20140929144423/@comment-7418318-20141001162319

Considering that defeating Tartarus and Zeref is in their own best intesrest and that they'll likely need Jellal's power to do so, I can see them going into a temporary alliance with him. After all neither Tartarus nor Zeref are likely to show them any mercy or allow them to join, even if they tried.

And if they know that Ultear was manipulating Jellal into doing all the atrocities, they might be a little more sympathetic toward him, since they know what it is like to be used by an "evil master". Not enough to befriend him or anything, but enough to not instantly backstab him the moment they no longer need him at least. Though I still wouldn't like to be in Jellal's place the moment Tartarus and Zeref are dealt with...

In short: I can see them working together until Zeref is defeated, but certainly not working for him.