Addiction to Male Friend. On what to do.

My mother joined this WhatsApp group last year and she, who was previously very reserved and never had friends and very family oriented , started getting very addicted to this group - this group consists of a men and women group , who were her 4th grade classmates. She became very close to one of the males in that group. It started off in the name of friendship and soon led to an affair. She used to text him regularly. She used to be very truthful previously, but started hiding things from us. She refuses to come to a counselor , psychologist or get any therapy. My father found some messages of her having this affair and confronted her after a long time. She was stopped from texting this male friend ever since. However she is still very addicted to the WhatsApp group and keeps convincing my father everyday if she can message him. Otherwise she threatens us of killing herself and harming herself. She refuses to get therapy, requesting you to kindly give us advice on what to do. Thanks.