Njgal, the possibility of them getting distracted is there , but it isnt a big one. if they are experiencing a stall time in theri marriage , perhaps friends and family should encourage them to make an effort into working things out.
Unless they have grown apart tremendously , not just physically but emotionally as well, I dont think they will take interest in anyone around them...specially when a person's marriage is in trouble, the last thing they look forward to is to get into another relationship.