Here is the order of events with respect to the flower:
Aerith gives the flower to Cloud.
Whether or not Cloud gives it to Tifa, it still initially represents the reunion between
“lovers” Cloud & Aerith.
Tifa has to inquire about the flower. Cloud doesn’t automatically give it to her without prompting.
Later, when Cloud interacts with the flower, he still associates it with Aerith:
Cloud doesn’t want to admit to Aerith that he gave the flower away.
After going to Aerith’s house, which is filled with flowers, Tifa confronts Cloud about the origin of the flower (ie: Aerith). As if she knows Cloud's heart is with Aerith.
Regardless of all of this, the automatic trophy for the "Budding Bodyguard" chapter is "Reunion":
In Japanese, the automatic trophy is titled
"REUNION FLOWER."