The term abuse brings to mind a burly guy threatening with fists like ham hocks, but it can also be meted out much more subtly, through words and innuendo. Emotional abuse is characterized by behavior that undermines your wife psychologically. It can include ridiculing her, scoffing at her feelings or opinions, or shouting her down. What you can do to stop her ex from this sort of behavior may be limited to helping your wife establish that she's not going to take it anymore.
Your wife has already broken up with her ex once, and she can do it again, particularly if there's no reason you must allow the man in your home or maintain contact with him. If they have children together, this recourse becomes more difficult, but it's not impossible. You can still minimize your wife's exposure to the man, thus robbing him of opportunities to abuse her. Arrange to drop the kids off yourself when it's time for visitation, so your wife doesn't have to have contact with him. To the extent possible, act as an intermediary between them regarding parenting issues, and make it clear there's nothing else the two of you want to discuss with him.
On those occasions when your wife can't avoid her ex, defending against his behavior begins with her, not you. Ask her how much she's willing to accept or can comfortably take. A comment like, "You never could master the art of being on time" might not bother her too much, whereas it crosses a line if he yells, "You're so self-centered! Can't you even consider that other people might be waiting on you?" Help your wife draw a mental line in the sand between what she will accept and what she will not.
What to Do
After your wife has determined how much she's willing to take, the next question becomes what she's going to do about it if her ex crosses the line. No one can abuse someone who isn't there or isn't listening. If you or your wife fight back, her ex wins – he knows he's gotten your goats. This is the sense of control he's looking for, so take it away from him. Your wife can tell him in a calm and reasonable tone that she's simply not going to tolerate such behavior anymore. If he denigrates her or shouts at her again, she will leave and terminate their conversation. The important thing is that she actually does what she says she will do – repeatedly, firmly, and without fanfare. If she always takes action, eventually it will become clear to her ex that if he acts up, she's simply going to remove herself from the situation.
What Not to Do
Try not to give into the temptation to treat your wife's ex the same way he treats her. The more of a reaction he gets, the more you've rewarded him for his behavior. Above all, don't lose your temper with him. If he feels challenged, particularly by another man, the situation might escalate into physical violence.