That person is probably just jealous your mom is happy and she is not. Your mother should quote the scripture that says Jehovah is a happy God and the one that says "happy are the ones doing the will of Jehovah". She should say that if she is not joyful it would mean she isn't being blessed by Jehovah. The best defence is a good offence.
There are people like that wherever you go, but the JWs do seem to attract more than their fair share of them. I call them fun sponges, they soak up all the fun wherever they go.