the .pck is actually the whole project. the executable is just the godot runtime. it is unaffected by whatever your project is. if the friend also has godot export templates, he can use that to open the .pck file by giving the file and the exectuable the same name
only other solution is to compile the engine from source code yourself. there is no other way around it. the executable will always be the same size, if you use the export templates. judging by your difficulty with existing file sizes, i think compiling the engine will be far more difficult. so you might be stuck with this. however, you could, reduce upload size a couple of ways. firstly, you could use “itch.io butler”. this program can automate uplaoding games to itch.io. butler will, in its first use, upload the full executable and the pck file. but for every other use, only the pck file will be uploaded. because that program only uploads files that have changed. secondly, you could try this website https://gotm.io
this website allows you to just upload the .pck file only and runs the game.