Do I tell my friend I think her relationship is destructive?

[Dollykazaver]

Or keep schtum and be there when it all blows up again?

We probably need a little more detail before being able to comment.

I think most know when they are in a destructive relationship.

He doesn't like her children, who have been through a rough time during her divorce. They're teens, their dad is doing the Disney dad thing after having behaved terribly. They can be rude and disrespectful and BF doesn't like it and doesn't want to spend time with them. This is the recurring pattern. She's torn in two.

I'm afraid that if I say something then she will isolate herself further, as she's very keen to make it work.

It doesn't sound like a particularly destructive relationship from what you've said. It doesn't make him a dick because he doesn't want to spend time with someone else's rude and disrespectful teenagers.

However, it does sound as though she needs to concentrate on her children's emotional health though.

It's fine to have a boyfriend and not involve him with your children. I've had 2 relationships since I separated, neither was involved with the children.