tldr at the end.
  
I think it's usually etiquette to leave out the bad aspects of an employee when you are called as a reference anyways, but I left out so much this case I felt like I was misleading.
  
Essentially, the bad employee (BE) wanted to be an operations supervisor. The job essentially requires you to be a walking dictionary of the operations manual, and barring that, know where to find the guidance to any question. I knew BE wanted to do this role, so I had her shadow my staff in that role. My supervisor tells me every time in our 1:1 that BE sucks, doesn't take to any training, and should not be in this position since she doesn't follow instructions. I encountered this myself, when she used to come to me to ask "how do I do this?" My reply being "if you want to be an ops supervisor, then why are you not looking this up yourself?" And set the expectation that she she be fielding other staff's questions, instead of coming to me and my supervisor with (didn't say it but they were) stupid ones.
  
One day, I get a call from my counterpart from another branch and asked me if I can answer some questions as BE applied to be an ops supervisor in his branch. I honestly didn't know what I can say, at that point, I didn't have a good opinion of BE. I just told him to ask me what he wants to know. I answered in very open terms like and withheld any very valid personal opinion like "ya, BE knows how to do it (but not well at all)" "BE does have this responsibility (but the guy overhearing the guidance learnt how to do it and taught BE how to do it and is now more responsible for it over BE)" and also, to be honest, maybe because the other Branch's manager was thinking I did not understand his question, was being kinda rude, but I mean... Not that guy's first reference call, so if he is hearing answers like that, I hope he got the subtext.
  
In any case, BE went to do the interview, never knew how it went, but during lunch, someone mentioned (scarcastically) how BE must REALLY want the job to be going back and forth across the entire city every day. BE then said BE didn't even realized and ask me to help BE withdraw the application. I told BE, it's an HR thing, BE should just call HR directly (I could have, but was annoyed at that point). 
  
Then at a company function later that year, the other manager had a chat and since BE quit to work for BE's father, I didn't feel the need to hide it and told the other manager he dodged a bullet. He was then kinda pissed off at me to say it was an asshole move to offload a bad employee on to him (as a joke but I can tell he meant it). I told him I'm sure his interview skills would have picked it up anyways, and we left it at that.
  
Tldr: didn't tell the other manager why my employee sucked when he asked, was told I was an asshole for offloading to him even though he already had the interview with my employee