A friend is having some serious issues when trying to connect to only my map

[Arbiter_Sol]

Just to cut to the chase, my friend is having some really strange errors on a gmod server only when it's running a certain map (the main one that I made). Apparently some other people are having this problem too, it's not framerate drop lag per se but more of a "walk for a bit, pause, walk for a bit, pause" kind of lag that often results in a crash. Loading into the server myself I have no errors and function fine aside from framerate drop since I use a potato, but others are also able to connect and play with no issues. Checking his console the errors seem to be many and random: stuff ranging from
Missing Vgui material vgui/hud/halo_swep_battlerifle
to
Attemped to precache unknown particle system "halo_beam_main"!
(this one shows up a lot) and
No such variable "$invulnlevel" for material "models/prifle/plasmariflestabilizerray"
Error: Material "models/prifle/plasmariflestabilizerray" : proxy "InvulnLevel" unable to initialize!

This only happens on one map, and when the server changes maps he's fine. I have no idea what causes this due to my limited experience with mapping and the source engine, so I was wondering if anyone could help me out here.

I have a google doc of his console from initial connection to just after loading in, right here.

I'm relatively new to mapmaking so I don't know if this is from the map's end or if there's some plugin or addon causing some issues.

[Alphaverse]

To find out if something is caused by the map or by other addons just remove all addons and test it again. If you still have errors you know what is happening.

This looks like some problems with materials. I think something was setup wrong or content isn't packed into the map...
Are there any errors when you compile the map? Just copy your compile log here and check: Interlopers.net