The solicitor is acting for the executors and so doesn't owe you a duty of confidentiality. Indeed the reverse is the case: the solicitor has to report such conversations to them. What you should do is instruct your own solicitor to write challenging the will on the ground of facility and circumvention. What that means is that your grandmother was vulnerable, subject to influence and not able to change her will as she was incapable of doing so. The solicitor may have to get a court order suspending the further dealing with the estate by the executors pending the court deciding the issue of competence. I hope that helps. Please leave a positive rating so that I am credited for my time.
Your mother can contest the will notwithstanding that she was appointed executor. She will need separate representation as the estate solicitor can't act for her in this.