Social Interaction Puzzle

I encountered a social interaction that perplexed me. I invite you to put yourself into this puzzle and consider your reaction. To do so, you'll need to fill the following roles with people you know:
- Cousin. Preferably of an age that them having a longtime relationship seems reasonable. (If you don't have such a cousin, you can substitute someone else. We're looking for someone you more-or-less trust, regardless of how close you are to them or how much you like them.)
- Significant Other. Your own sweetie. Could be spouse, long term relationship, or even your newest fling. (I imagine longer relationship might help the experiment, but who knows.)
- Friend of Significant Other. Someone you might know, but they were initially your SO's friend. (Note - This person should be in a different social circle than your cousin. We're looking to create some horizontal distance.)
- Friend of Significant Other's Partner. As with your own relationship, this could be long lived or recent.

The more of these people who actually exist, the better. However, let's say your partner's buddy is presently unattached... you can give them a fictitious spouse.

Here's the situation. Remember, this is all imaginary, but try your best to assign people to the roles and see how it feels.

Your cousin breaks up with their partner (henceforth "the Ex"). (This could be divorce, called off engagement, or a break up, but this is a fairly major event.)
Your cousin shares some private details regarding the Ex. (We'll come back to this later.)
You are happily in a relationship with your Significant Other.
Your Significant Other has a long-time friend.
This friend has a partner.
You happen to see a photo (on Facebook or something like that) of that couple (friend and partner) along with Someone Else... and the Ex.

Hypothetically, what information would your cousin have to have shared with you regarding the Ex that would cause you to feel compelled to reach out to your Significant Other's Friend's Partner, to strongly encourage them to warn this other person about the Ex?

Here are some possible examples of things you might have learned:
- The Ex cheated on their taxes.
- The Ex cheated on your cousin.
- The Ex cheated at Monopoly as a child.
- The Ex has a drug addiction.
- The Ex has a gambling addition.
- The Ex has a pornography addiction.
- The Ex has a Candy Crush addiction.
- The Ex has a Jane's Addiction CD collection.
- The Ex has poor grammar.
- The Ex has poor hygiene.
- The Ex has poor circulation.
- The Ex is a poor tipper.
- The Ex is a poor lover.
- The Ex is a poor sport.
- The Ex is simply poor.
- The Ex once hit a hole in the wall.
- The Ex once hit a hole in one in golf.
- The Ex once hit a whole bunch of garden gnomes with a riding lawn mower.
- The Ex once killed a man to watch him die.
- The Ex once poured out a perfectly good bottle of wine.
- The Ex once thought Star Wars: Episode I was the best of the series.
- The Ex once mowed their lawn naked.
- The Ex once showed up at a funeral in a clown costume.
- The Ex roots for [rival sports team].
- The Ex feels [not your favorite candidate] would be a good president.
- The Ex doesn't always wash hands after handling raw meat.
- The Ex drives alone in the HOV lane.
- The Ex doesn't come to complete stops at stop signs.
- The Ex doesn't wait 30 minutes after eating before swimming.