Employers are often called by a former employee's prospective employer for a work reference. Care should be taken in giving work references, especially for a less-than-satisfactory employee.
Some questions and responses are fairly safe:
Often an employer doing reference-checking will volunteer what the employee told them, and let you comment or correct.
You need to be careful about saying negative things, that may border on defamation. Even not giving a reference, when doing so for other employees, can be as bad (according to court decisions).
Here are some suggestions to make reference-giving manageable:
Of course, you can try turning a negative into a positive. For example, if a person cannot be depended on for independent work, you might say, "works well under close management supervision."