To think my friend is making a HUGE mistake and she should leave well alone.

I love this. Poetic justice

This happened to friends of my parents, the chap had an affair and went off with OW, the wife of the original couple ended up in a relationship with the husband of the OW.
As far as I know they are all still together, the couple don’t mix but I think it probably happens more than people think because the spouses that have been cheated on help each other come to terms with what’s happened as they both know how each other feels.

Why is it your business?

Your poor friend has been through a terrible time. She's happy with the new chap. It might work out, stranger things have happened! It might just give her back some self-esteem and confidence after being devastated by being cheated on. Support her and be happy for her and also be there for her if it goes tits up! That's what friends are for!

Good for them. I hope they have a lot if fun and find true happiness with each other. Any relationship can be messy or end badly. Children should always be protected but who cares what the original cheaters want.

This exact same situation happened with two couples who were friends of my mum’s many, many years ago. Both couples were very happy together and were married for at least a decade back then. They’d be very old or dead now.

I know people this happened to. Two couples who were friends each had two kids and the woman from one couple and man from the other had an affair. Both marriages broke up and the first couple moved in together. Similar scenario to OP's friend the other couple supported each other and eventually got together too. Umpteen years later both couples are still together, kids all grown up and grandchildren in the mix. The couples are not friends at all. The couple I know are the ones who were left and in actual fact they are a better fit for each other than their original partners, have similar interests, life goals etc. At the time it all happened I thought it was a disaster waiting to happen but I was wrong.

My ex neighbours swapped their partners and have been.married for many decades.

This happened at a school I taught in. All four kids, two from each family. were at the school. It was a bit strange seeing Mrs A with Mr B and Mrs B with Mr A at school events but the kids seemed to take it in their stride. I bumped into one of the couples recently and they're still together 25 years on.

Why is it any riskier than any other new relationship? They almost certainly bonded over the joint betrayal but that doesn’t mean they don’t genuinely get along aside from all that now.
I rather think it has a lovely poetic justice to it. Even more so if the exes are raging.

Nope. Only one child. Aged 4. Very clear.

Not your circus...

IMHO, take your friend out for a cup of tea and sit her down.

Tell her that you think that she needs time to process the end of her marriage and should maybe slow things down. Also tell her, it is absolutely her decision but as a friend looking out for her you've provided your opinion. And underline that this coffee chat is the absolute last time you are mentioning these concerns...you will forever hold your peace.

Finally, as a pp said, keep the kleenex handy because this situation has a lot of baggage and is more likely than not to go sideways.

When my cousin found out her husband had been cheating on her for SIXTEEN YEARS, she briefly dated the husband of the woman her husband had been cheating on her with.

As someone up thread said, it's a trauma bond.

It doesn't say anywhere that the friend has a child? There's a 4 y/o belonging to the OW.

I also know someone this happened to! No kids though. I think part of it is trauma bonding, part of it is that’s the first person of the opposite sex they are in close contact with post heartbreak. It’s like the Titania effect!

Her ex and his OW really can't be upset. It's honestly ridiculous.

This happened to Shania Twain.

Twain met producer Robert John "Mutt" Lange after he heard her original songs and singing from her debut album; he offered to produce and write songs with her. Having spoken on the phone for many months, they met at Nashville's Fan Fair in June 1993 and became close. They were married on December 28, 1993, and had a son, Eja (pronounced "Asia"), on August 12, 2001.

On May 15, 2008, it was announced that they were separating after Lange had an affair with Twain's best friend, Marie-Anne Thiébaud.

Their divorce was finalized on June 9, 2010.

On December 20, 2010, it was reported that Twain was engaged to Swiss Nestléexecutive Frédéric Thiébaud, Marie-Anne's former husband. They were married on January 1, 2011, in Rincón, Puerto Rico.

I know two couples who did this too!

The original cheating couple were both fed up after a very long marriages rather than being intrinsically attracted to each other. Both families were good friends ( through the kids) did annual holidays and weekends

away.
The cheated on couple broke up but the original ones are still together. Do do one weekend at theirs altogether and then the next at their respective other parent.

Flipping heck. Real life wife swap!

In one of Adrian Vaughan's books, he was working in Oxford in about 1970 and his 'landlady' swopped with the woman next door, first he knew about it was when she came in with his breakfast/

He's writing about his career as a signalman so doesn't go into much more detail

I'm sure this happened with Shania Twain. Still lasting.

Do you think they are genuinely happy together or is it more of a trauma bond between them?
I would be wary too. It would be lovely for them both to find happiness, and there would be a justice to it, but it sounds like they are moving a little too quickly.

Know someone who this happened with aswell, the original cheating couple split up due to.. surprise surprise…more cheating. But the other couple are still happily together 20 years later

Thank goodness there's only 1 child