Is it ethical to ask about the professional profile of a candidate at another university before a job interview?
In most situations fitting the pattern you describe, you should not ask your colleague.  There are several possible cases:


If the candidate has requested their application be kept confidential, you absolutely must not reach out to them or to anyone else the candidate has not listed as a reference.
If the candidate is currently in a tenure-track or other permanent position, I would not ask without checking with the candidate first, because outing them could be very problematic.
If the candidate is currently in a soft-money or other contingent position, then I would similarly be very careful about asking, but the consequences are not as dire as in the previous case.
If the candidate is in a postdoc or similar position where they are expected to be looking for a permanent position, then asking is a violation of the implied confidentiality of the job application process, but it is a common violation.