Install dbeaver or something. Connect to that database with the sysadmin credentials from your local machine.I don't know what that means. I know troubleshooting can be pretty interesting and I'll let you know what our experts find out, but this is really just to give a general outline what I do and which MOC we should tell our boss we need is mainly what I'm after.
in unrelated news, the other guy managed to get back into his SQL server (I overheard on the phone him talking to their actual SQL expert and I could actually hear him shake his head as he told the guy that he should just reinstall the whole thing and pay attention this time) and it was smooth sailing.
If this is successful then you will eliminate the possibility of it being an auth error with you software. Assuming it's using the same credential.
I've never attended MOC or anything. I just learn by troubleshooting and youtube tutoroals. The Microsoft official trainings should be good enough no?