You have two choices either ignore the message or respond.
A simple text saying thank you for thinking of me but I am not able to go out this weekend but will call you if I need you. This is a stalling non threatening message that gives you breathing space.
However you do not owe them or need to be controled by them. How you chose to respond relates directly to what you want to accomplish. If you wish to remain non threatening and fading then you have to live as a JW would live minus meetings and service and not appear to question the society. If your reasons for non attendance are personal weaknesses e.g. working too hard or medical condition then that is ok. If your reason for non attendance is non belief and you admit it then that is the death knell for your fade.
with witnesses try to avoid probing conversations. If you are caught by surprise always have a strategy to force the conversation to be kept short either an appointment or somewhere else to be or illness the more contageous the better. focus on what you can say rather than what you can't. E.g.
Thank you for your concern, I am sorry that now is not a good time to talk, I am dealing with personal stuff right now, I know where you are and will certainly contact you if you can help.
Fading is an art and a lot harder than people imagine since JWs are natural snoops and able to pick up any nuance that shows that someone is not on message. Be careful.