A few general points from a single:
If you have information which you are not sure about (such as a medical issue), ASK the single/family when you are first asked to be a reference or you can broach it gently later on (“I’m very happy to answer questions, I’m sometimes asked about ______, is there anything specific about it that I should know?” or you can ask them to update you about several general things, and include that particular issue in the list).
I once had an idea for a shidduch and was aware of sensitive information on both sides (one side pertaining to the single, other side to the single’s family). I asked a shaila about what to say and while on the phone, I asked if it would be ok to give the name and info to the side I know better (the side with more sensitive info, relating to the single specifically). He was very happy with that idea until I got to the next issue, that I was unsure if they would say everything. He was very uncomfortable at that point and did not give me the go ahead.
Any friend (or other reference) wants the single in question to be happily married. Discovering a bottle of pills, cigarettes etc during Sheva Brachos is not generally helpful in starting a happy, healthy marriage. Despite any temptations to “help” someone by not giving info correctly, you are providing an extreme disservice and finding a great way to hurt them (as well as likely going against Halacha).
Note to singles: If you’ve been in the system for a while, you may want to check or change your references.