I suggest having your legal counsel handle this matter, since it risks litigation.

Having said that, defamation is not one of the crimes for which a prosecutor can issue a decision to block access to the web page. The hosting company is not liable for the acts and violations of content providers (the second company, in your case). They also are not obliged to reveal the identities of the users - they seldom have that information anyway. The hosting company which your client has contacted may at their discretion choose to cooperate, but again they are not obligated. You may also contact the second company directly and demand the removal of the defamatory content. If you do not receive any word from either of the host or the user within 2 days, you may file the same demand at the court. The judge is required by law to decide on the matter within 3 days, an ultra fast process by any standard.

Hope this helps, but please note that you should definitely let your counsel handle this case, and this web site is not intended to be, and you should not rely on any materials on here as, a source of legal advice.