For all intents and purposes, they already have, at least once.
The entire 30-50 DRK questline showed they were incredibly close to having an actual mental breakdown, if Fray appearing wasn't already a case of it. Then after Haurchefant's death, 1 week passed without us doing ANYTHING. Exactly 1 week without us controlling the WoL. That's how hard they took it, and it's somewhat implied by dialogue choices that the rest of the 3.0 storyline was us on a revenge trip out of grief. Then there's the level 60-70 questline pretty much HAVING the WoL have a mental breakdown, even ending with the job trainer asking if they need to talk to him, to let it off their chests.