If there is a smoke, then there's fire.
If a company is getting so many negative feedbacks on the internet. I don't think it is wise to do business with them.
And based on your job description "
SEO, PPC campaigner and also web analyst", I don't think it's your job to clean-up the reputation of your client.
The best thing they can do is to provide quality and honest services/products, and then positive feedback would come naturally but gradually. But then, it's not your job. It's their job.