No matter the provocation, it is rarely a good idea for employers to go too far in their criticism of former employees.
It is perfectly in order to provide a less than glowing reference (or indeed no reference at all) provided that any reference is a truthful and accurate account of the employee's performance/attendance etc. On occasions, when relations between employer and employee break down, the relationship may come to an end via a compromise agreement which will often include an agreed form of words for future employers regarding the employee. Notwithstanding this, the temptations for the ex-employer to speak their mind can sometimes be overwhelming.
A good example of why this is not a good idea is found in the recent English High Court case of McKie v Swindon College. In this case Mr McKie was a good employee and received a reference confirming this when he left the College for new employment. His new employer had dealings with the College in the course of which the College's new HR Director sent a email containing serious and damaging allegations against Mr McKie. As a result of this, Mr McKie lost his new employment. The Court found as a matter of fact that the allegations raised by the HRD was not only false but also sloppy and slapdash and found the College liable to Mr McKie for damages caused by their negligent misstatement.
The lesson for employers from this case is clear. Regardless of whether it is through a formal reference or an 'informal' chat over the phone or other means; an employer should take care when providing information regarding an ex-employee. A statement that is false, malicious or negligent can land an employer in hot water.