Could be a religious group that left them. This weekend is Palm Sunday and next weekend is Easter.

That said, if someone got in your friend's apartment and only left a flower, I'd rule out the homeless because they'd probably cause more changes while staying (e.g might eat food, take valuables, use the shower, leave trash), and if they're smart enough to avoid showing any changes they wouldn't leave a flower behind inside and risk tipping yall off and losing a good spot.

I would think if it was anti-LGBT motivated there'd be vandalism or damage, and they wouldn't leave behind a symbol of peace/rebirth.

So, probably a creep. Or a prank.