You have written that "She is creating unnecessary noise and malicious talk about company". Two things emerge from this sentence. One is that she is frustrated. You need to identify the cause of her frustration.
Whatever may be the cause of her frustration, slander against one's company is certainly not acceptable. However, she needs to be nailed properly. For this, you may send a decoy to the client site and tell him/her to provoke this lady. However, this has to be done subtly. On provocation if she smears her company then that person should make audio recording clandestinely. Once the person returns from the client site, he/she may submit the audio file to the HR Department. On the strength of the evidence, you may take necessary action.
Nailing the backbiter is one thing. Nevertheless, this also appears to be a organisational communication issue. Do employees get chance to express their views? What are the provisions for upward communication? How many times senior management personnel interact with the lower level employees. Do they take feedback only from their immediate subordinates? Do you conduct skip-level meetings? You need to find out means to improve internal communication in the company.
Were there no problems with her till two years? How could know about her behaviour issues?
Anyhow now you can start keeping a watch on her activities and if repeated then take action.
In the initial stage I suggest you arrange a personal talk with her through her HOD or talk with her personally in the presence of her HOD (can be on video conference or concall as you said she is on client site),on what her problem is, when everything is given categorically by the Management for her like increment allowances etc.
Talking bad about employer to an outsider is a big No No.
What is the source of information that she is speaking bad about the company?
What exactly is she supposed to be saying bad about the company?
Once you understand the message she is speaking about, call her over to the company and counsel her adequately and then monitor her response and whether behaviour has improved or not.
Try and also see whether her complaint has some sense or is it mere ranting with no basis?
The behaviour issue of a lady employee , regardless of reasons , has serious implications oncompany's reputation and cannot be allowed to continue.
Ascertain the exact facts like what ,whom , where and when she spoke against the company and confront her in one to one meeting. Tell her to change her ways or she will be changed. No compromise with company's reputation for any reasons.
Talking irrelevant and spiting bad against the company is considered as misconduct. As per you she is instigating other employees aginst the establishment and misleading the mind of employee are gravious nature of conduct. You also alleged that in recent past she misbehaved with another employee who went to client site for final round of discussion for his job.
You are saying that she has not having any issues till now and remained associated for last two years. It is definite that something is there for which she is unhappy and spiting venom. You call the people gather informations with whom she is deleberating her talk. If you follow the discussion you can able to get the hints. If the things have crossed the limit, asked the staff to give a written complaint to whom she recently misbehaved. On strength of the complaint summon her to your office for discussion. You record the discussion but before hand you need to prepare fully on strength of information gather from different sources. Do not let the things as it is. You also follow the guidance provided by other members of the community.
Proceed with caution. An employee showing normal behaviour for two years, all sudden jumps like Jack Out of the Box! There is something more deep rooted!
A whistle blower might help, but avoid a decoy to look into the issue. The best option the HOD/HRM/Management need to have a direct talk with the staff: something may have been simmering since long, which went unnoticed, till it reached boiling point!
When something goes wrong, a good management principle is to blame the management itself. Management often rediscovers, the what and how of any issue, and then works patching up the blame/fault.
No sane person, without reason, would go blowing a trumpet against the source of her/ his bread and butter.
Please check, if you begin pointing the finger at the staff, remember 3 fingers are pointed towards you.
Hope this perception helps
Arif ur Rehman