On NoPayPOKER, I believe good sportsmanship says you should post in the lobby that your opponent is away and slow play for a while to see if they can make it back. For the most part this is a friendly site and we all play together every day so treat others like you would want to be treated, even if they come back and win. It may just be me but I want the people I play with every day to know they can trust me.
On other sites you cannot call out in the lobby but you can slow play some to give them time to make it back.