In Germany (and possibly in certain other countries) there is a special language people use in written references to issue a positive reference (others aren't permitted) yet include certain information "between the lines". There are books on how to write these things and how to detect them.
Same in the US. Basically you can either give someone a glowing review, or you can say, "yes, <person> worked here for <X amount of time>".
What you can't do is anything that hinders a person from getting a job, but it doesn't mean you need to lie and say they were great.