I have a friend whose been in a 'relationship' with a man for 11 years, whilst he's been with someone else. They met before he met his partner, stopped for a year or so when he met her and my friend met someone but have continued it ever since. My friend is now single and has been for a long time. They message pretty much constantly in the day when they are in work & when his partner goes to bed at night. They sleep together at least twice a month and apparently 'have a bond like she's never experienced with anyone else'. I've given up even telling her how wrong this is and how much hurt she would cause to his partner if she were to find out because whilst she realises she's doing wrong, she says she can't and won't ever be able to end it unless he does. I've never been cheated on, as far as I'm aware previously so can only imagine the devastating impact it can have. I came across the partners Instagram a few weeks ago & her photos scream happy, committed relationship, lovey dovey posts from her calling him her person etc. I felt awful even seeing it but I'm not going to get involved.