Did you alliance member cheated? Why would you recommend giving them less rewards? If it's for AW, Did they do the 5 fights minimum requiremen? Which seaon rewards are you referring to? Need more context here.
Are you talking about AQ? Which season rewards are you referring too? If he joined later, you can't claim rewards that the alliance already achieved. So any new rewards he can claim. AW season is over but if you did the 5 fights minimum before season ended than you would qualify otherwise he can't. It's to stop players from jumping from one alliance to another and claiming multiple rewards.