If your ex-girlfriend gets her marriage annulled or she gets divorced from her husband, at that point, there’s no reason for you not to start dating her again. But with caution.
Remember — you could be the guy the guy she’s divorcing now. Your friend wants to get married, and she’s able to do that — she’s proved she knows how to get that done! But she may have been impulsive in marrying her current husband — especially if during the marriage she’s telling you she made a mistake and wants to be with you. Think about it from her husband’s point of view. What she’s doing could really be seen as cheating. If anyone is going to make a marriage work, they have to take it seriously. I don’t think she is taking hers seriously enough.
For now, if I were you, I’d back off and let her be in her marriage. Don’t be involved in the break up of your ex-girlfriend’s current marriage. It’s really none of your business. When she married her current husband, she should have let you go out of her life as a romantic interest in order to commit to her marriage. That she didn’t do that — that she wanted her cake and to eat it, too (the same way she wanted you and for you to move to her city), describes who she is. You should really take note of that.
Relationships survive and flourish because both people in them respect their partners and are willing to sacrifice and accept sacrifices in order to make them work. So if this woman is someone who you think will be able to do that for you, then by all means, you should consider dating her again. But for now, no contact until she’s a legally single woman. She has to decide to stay in or leave her current marriage because of herself — not because of you.