The others have provided some good points for you. Gaining weight doesn’t mean this client is not achieving his/her goals. You should take in consideration the BF% because that is a better reference to go by (I hope you did a BF% on the initial assessment). Also, if the client has experienced improvements in his/her overall fitness level then you are doing your job. If BF% has gone up as well then you will need to reassess the training program and also find out if something else is going on when the client is not training with you. Diet can also play a big part here. I hope this helps.