Anything negative at all, and a lot of positive things could be interpreted as partially negative, exposes you to risk if the employee doesn't get the job.
I am not a lawyer.
There are various ways to get around this when getting or giving references.
Outside of that, the best thing to is to confirm employment with dates. Oddly enough, some people will list a reference from where they were fired for misconduct. I'd bet that said person doesn't even understand what constitutes misconduct, why they were fired and how to avoid it in the future.
Call or email, leave a message asking to be called back IFF the candidate was exceptionally strong. Assuming you aren't a competitor, you'll probably get a call back if true. If the guy was meh, it is a nice way to pocket veto.
For when you do employee referrals, if employees are getting pressured by lames to refer them, let them do no-op referrals. Default bring meaningless, and "strong referral" being a real referral. Google, Facebook, etc do this.
It would be exceedingly difficult to successfully sue someone for failing to return a call or email from a random outsider. Plausible deniability.
This is also one of the cases where being part of a "mafia" is awesome -- you can actually call up and get unvarnished opinions, or at least, cagey "I don't think I would" "That might be difficult" etc.
Your suggestion would basically just screw over any person whose boss had more pressing matters on his plate.