If asked to give reference "by surprise" (i.e., friend hasn't asked if it is OK to give name for said purpose) then (a) one is happy to speak highly of the target individual or (b) one gently refrains from doing so giving whatever appropriate reason (I am not qualified, I don't think it appropriate blah blah). All references should be given in writing. At NO TIME, however, should one be lulled into giving a bad reference...if someone is merely a colleague then it is unlikely you are qualified to judge and your views will reflect worse on you, if you are former "manager" or employer then you can and should confirm terms of employment and that they were satisfactorily completed (else person would have been let go)...
JMHO
