You're talking about debt in Euros right? Irish debt on an irish house?
Herself and her ex husband are jointly and severally liable for the debt. That means that if one of them can't or won't pay, the other one is obliged to pay all.
What that means in practice is that the bank will chase and pursue the one that is easiest to chase and pursue. So if one party has been paying something, is living in the house or dealing with tenants, has been engaging with them then that is the person they will chase.
Is her ex husb still living in the house? She will only start hearing from them if he stops engaging completely. And in the event that the bank hears from neither of them the bank will eventually repossess the house. So it all comes down to really, is her ex-husb still engaging with the bank and paying something?
To be honest I'm surprised that the house wasn't sorted out during the divorce proceedings? It's the kind of thing that gets sorted during a divorce normally?
Also if the house is in the Dublin area, or close to a city you may find the negative equity is less than you think and/or fast decreasing. She can check the property price register to see what similar houses in the same estate or area are selling for.