If he's already running other mods with D2SE, then it's certainly harder to figure out what's going wrong. For a start, maybe just quickly try installing the mod again (shouldn't take more than a minute), to see if something changes:
- Make a new folder in the Mods folder.
- In that new folder, extract the Core Files as well as the version of choice (you should end up with about 30 files).
- Maybe double check that the Configure Glide settings are correct for him (I believe 1024x768 is the only working option for HighRes, but I don't have much experience with it). I think you must use Glide if you're running HighRes.
- If he's running with the Jan15th update (important for HighRes Maggot Tunnels among a few other things), then check that the -direct -txt checkboxes in D2SE are checked, and that he's placed that Data folder into the Huv1.7 folder.
- Since he's running into a version conflict joining the realm, maybe double check that he's logging into Arimyth Gateway, and that on the D2SE menu he selected "Start Plugin" or double-clicked the HU banner in the list.
If you figure out the problem, feel free to report back with the solution