"Inform on" only means one thing to me, which is slang:
A criminal might "inform on" another criminal.
Criminal A tells the police about the crimes of Criminal B, and in return, the police release Criminal A from jail. Criminal A is informing on Criminal B. Criminal A is an informant. (...a police informant.)
But YES, context is very very important. I always tell my young son, "I need more information!" when he give me half-sentences.
@Andrew is right; I had assumed "inform on" was slang in that sense, but it is indeed in the dictionary:
verb - inˈfôrm/
give incriminating information about someone to the police or other authority.
"people called a confidential hotline to inform on friends, neighbors, and family members"
synonyms: denounce, give away, betray, incriminate, inculpate, report,
...and is used more widely than I figured according to these Ngrams (which keep surprising and fascinating me!) And, "blow the whistle" is considerably more than "inform on" in the USA, but other way around in UK.
I guess I assumed it was slang because it's a phrase mainly used by criminals... :)