Fun Fact Question about Marriage Couple?

Sarah probably would have had to have John declared legally deceased, making her a widow rather than a divorcee. But whether she divorces Fred to go back to John would be up to Sarah. Yes, she'd have to divorce Fred to remarry John once John reestablishes his identity as a living person. Or perhaps she'd be like Helen Hunt in 'Castaway' and just stay with the new guy. 

divorce the first dude, nothing wrong but he disappeared for 6 years. 

In castaway Tom Hanks wasn't married to Helen Hunt, but if he was, per your question, Tom would still be married to Helen and Chris Noth's marriage to Helen Hunt would be voided because you can't be married to two people at the same time (in USA).  Would Helen Hunt stay with Tom?  Probably not since she has emotionally moved on from him and fallen in love with Chris (whom is also the father of her children).  Thus Tom would be like any other dead man coming back to life after 6 years, trying to find their place in a world that has moved on.

In this case, no one has done anything wrong. Whatever happened was not planned or premeditated. 

It's just unfortunate.

Three of them can get together and work it out among themselves. They can go with something that best suits the situation.

How is any of this fun? In any event, John can be declared legally dead after 2 years without contact. John's reappearance would not in any way alter her marriage to Fred, so, from there, its entirely up to Sarah whether she wishes to remain married to Fred, or divorce him and remarry John, but her divorce to Fred would require the same equitable division of assets as if the divorce was motivated by meeting someone new

Your scenario is the plot of 'Cast Away'.

Sarah could not have married Fred without 

some kind of legal declaration regarding John 

(she would have had to have John declared 

dead in order to be able to marry Fred). John 

at that point is already an ex, regardless of his 

later showing back up. It has absolutely nothing 

to do with his doing "anything wrong" to Sarah 

(you seem to be confused about what being an 

'ex' means). If she chooses to return to John, 

she will have to divorce Fred (so Fred becomes 

an ex). If instead she chooses to stay with Fred, 

John was already an ex. The solution to all this 

is entirely up to the three of them to figure out -- 

there is no "best way", as clearly one or more 

of them will most likely be quite hurt by the 

decision, whatever it is.