Cheating = Divorce. Right ??

I have been married for almost 7 years. I got married at the young age of 22 and I have never cheated on my husband. I married him because he was and still is my best friend. My love for him after all these years has never decreased it only got stronger.

Why is it people find "the one" or so they say and cheat on "the one" ?

I have come in contact with someone has been married for 2 years. Their relationship was always rocky. Friends and family never understood why they got married, but that's what they wanted and we all supported it.
The bride in this particular relationship has 2 children from a past relationship. The kids are 2 boys 9 and 10, so they know and understand everything that goes on in the house. The bride also has 2 more children from this particular marriage. These 2 are 1 boy at the age of 3 and 1 girl at the age of 1. Lucky for them they don't completely understand.
The husband is a untrusting, mean, temperamental man who feels that he works all day so when he gets home he don't have to be a daddy or a husband, he just hangs with his friends.
The untrusting husband for years has always accused the wife of cheating. During fights he would just pack up and leave and come back when he feels like it.
Obviously this IS NOT the PERFECT marriage, however it is the marriage they both chose to be in.
One nigh 'husband' starts and argument and leaves with his clothes.
'Wife' calls me crying as usually that she does not think that 'husband' is coming back. My opinion to that was, "He's been looking for a way out for a while so just finally end it". After days of them not talking and her crying to me she calls up and acts like nothing happened and they are living happily ever after.
About two weeks later 'wife' calls me and says I slept with my ex. Ex being her 2 oldest father.
One of the wife's friends tells the husbands friends and husbands friend tells husband. Wife says she didn't do it and keeps that story up for 2 weeks.
If your going to lie you should stick to it, right ???
NOPE!! She confesses and now husband wants a divorce. Everyone around them thinks she did not do it. Only a select few know the truth. Now a wife who does not work and 4 kids are going to have nothing because a pointless sex. Oh did I mention wife is playing the victim.
Is it mean that I think that they should just divorce and get it over with. They never get along and it's going to happen sooner or later. How can one trust after one strays?? I don't think I could.