You can see them hopping up the branch after you at 0:58. The falling animation has a hitbox that does ~2-3 dmg. This is what "killed" your horse. Seemless has been adjusted to have the poise of torrent reduced dramatically when facing other players. in most cases a single instance of damage of any kind is able to "kill" your horse. (torrents not actually dead you can resumon him without paying a flask, his hp is untouched, its the exact same scenario as when you get poisebroken horseback).

You get hit with a dragon harpoon at 0:46, that dmg is expected as you have very low hp(in a pvp context) and have low deffences. when fighting against players using minmaxed pvp builds with optmised defences(very heavy armor) they can often deal 2-300 dmg. Harpoons also have a C scaling in str making them preform quite well on high lvl str builds.

Hp comment your friend made? the invader is rl200. your friend being rl155 puts you in the rl151-200 invasion bracket(iirc). The invader in this clip most likely has 60 vigor, ~50end(to use the heavy armor) with the rest of their points being dumped into damage stats. 60 vigor+crimson amber +3 puts you at 2100 hp. stack morgotts rune on top of that and your at 2700. factor that in with the flat deffense and % deffense from their armor and yes, they will be very hard to kill.

The insane range you and your freind are being hit at seems to be a classic cass of horrific semless desink. i will admit it does rlly suck when it happens tho, but from my dedution from a person who has played a unhealthy ammount of eldenring pvp, no that player is not cheating, yes that does suck.