Before I gave him the letter, he just stayed in the room looking for the letter. But after I gave him the letter, he just went to Andrea, who would reject him, then Hector just jumped into the river.
So how could I make an opportunity for him to use cocaine, and where?
You give him his lost love letter then kill Andrea by pushing her off the balcony while she reads the letter and Hector is standing just beneath. To get this you have to wait till the both take their positions and then make the kill so Hector sees her falling and assuming that she commited suicide.
Then after a while he goes back to the mansion and sniffs coke. If you poison it beforehand he will die of the poison.
The cocaine you need to poison is on the pool table in the Entertainment Room
I did as you told and it worked! Thanks!