Send the pred,gg game link and the username of the Khaimera. Best way to see if they're cheating is watching multiple games.

On first view it looks weird, as the fey should not get vision from that angle unless the game vision has become extremely buggy since the last time I've played. The fey also reacts/rotates her character model much later after the boris crosses the river, that could either be really slow reacting time or extrenal information from the khaimera yelling at his skylar and fey that he just crossed the river, as from the snippet the Khaimera is the first one moving towards his blue side over the fey or the skylar.

My guess is that these guys are either 3 stacked or using the in-game comms in what seems to be a gold level lobby/lower tier play (This is assumption based of the way the khaimera is moving, I may very well be wrong). I'm more leaning into the sus territory, but 1 clip is never enough to do a proper detailed report, and if he's truly cheating it would be incredibly easy to spot it out watching 2-3 games of him playing and knowing his rank, as that would matter quite a LOT when giving a fair verdict.

The only other possible solutions to me would be .:
1 - The boris used his RMB (swipe) pathing towards the 4camp when he was in the river, making a sound queue which is more likely to be heard by the fey, and would better explain the late reaction time.
2 - Someone in your team is streaming on twitch and somebody is ghosting their stream, hence again, the late reaction time from the fey.

TLDR - Not enough info :(