Should I tell him or stay quiet?

[bluebell9]

My DP found out that his ex (and mother of his children) had a fling with his best friend. It apparently happened months after they split, but my DP had suspicions about them towards the end of the relationship and other things point to them at least having an emotional affair before they split. The ex claims the friend caught her when she was vulnerable from the breakdown from the relationship and he chased her, she's never really taken any responsibility for her actions.
My DP hasn't spoken to his friend since he found out as he suspects his ex has lied about things and he doesn't want to sour the relationship with his ex as they need to get on for the kids sake, by finding out the full truth and that he's much happier now than he was in that relationship anyway so it worked out for the best for him.
His ex was and continues to be manipulative, but my ex doesn't always realise hes been/being manipulated.
Unfortunately, my DPs Mum told me something about the ex that happened while she was still with my DP that shows how manipulative she is but would be proving that the affair happened before the split and that the ex did the chasing. My MIL said she wouldn't tell my DP as it would only make the relationship with the ex worse, but my OH asked my yesterday if his Mum had told him anything about the situation that he didn't know. I hate lying to him but I don't think any good would come of it by telling him, apart from maybe opening my DPs eyes to how manipulative his ex is.

What would you do?

[ImperialBlether]

Once someone else knows, then you have to tell him, I think. I would want to know and would hate to think of people talking about it behind my back.

[happypoobum]

but my OH asked my yesterday if his Mum had told him anything about the situation that he didn't know.

Really? Does he have psychic powers?

[pinkyredrose]

Why did he ask you that? Seems rather strange. And why is he so desperate to know what his ex did when so much time has passed after they split up?

[Lweji]

The problem is that if you tell him, then you have to end up saying how you found out. And he may get cross with his mum.
Not sure about this one.

[bluebell9]

He knew his mum had been talking to me about the ex and I said it made it difficult for me to like her (the ex), so he asked why and did I know something he didn't. Although the split and fling happened a while ago, he only found out about it a recently.

[ChuckBiscuits]

but my OH asked my yesterday if his Mum had told him anything about the situation that he didn't know

Do you mean - your partner asked if his mother had told you something that he didn't know about? Why would he ask you this - presumably he thinks you and his mother are talking about it?

Why not tell him if he suspects something to go talk to his mother.

[furryminkymoo]

Tell his mum to tell him. Gets you off the hook and the truth is out?

[TheNaze73]

Not your news & not your job to tell him either

[Costacoffeeplease]

Why should you tell him? Tell him to ask his mum if he thinks she has information

I don't really see why he's still bothered

[Lweji]

Yes, he should ask her, not you, in that case.

[AnchorDownDeepBreath]

Unless you've made it really obvious that you know something he doesn't; I wonder if this is a test of your honesty...

Bit coincidental that he asks right after his mum tells you, otherwise.

[Notgreattimes]

How long ago did they split up ?

[HeddaGarbled]

I would just forget about it. Your MIL is stirring. It's only gossip anyway. Maybe she's trying to pally up to you by bitching about the ex. Just say that to your partner if he asks - "she's just bitching, I'm not interested".