Batsman did not field

"The opposition gave in their list of players to the umpires, lost the toss and fielded throughout our innings with only ten men - their captain said their missing player 'might turn up later.'
Then they batted. When their ninth wicket fell, we all started to troop off, thinking the game was over. Suddenly, the missing eleventh man on their original team list walked out and batted for 3 overs to save the match. Is this fair?"

It may have seemed somewhat unusual, but what this team did was not illegal according to the Laws of the game. There are Laws that govern when a bowler is able to bowl should he arrive late for a session of play, or what happens if a batsman is forced, or indeed chooses to retire, but no Law insists that a batsman must field before or after he bats.