Two different computers downloading same spreadsheet - data displays differently

I work for a company in the United States. One of my clients is in Montreal. Whenever the client tries to download a spreadsheet from our network, she ends up with a bunch of data bunched into Column A, instead of spread out properly over multiple columns:

When I download the spreadsheet from the same source, I end up with the correct format:

When the client emails her downloaded spreadsheet to me, and I open it, I get a third data layout with semicolons:

Ultimately, the client needs to be able to make changes to her spreadsheet, then re-upload it to our network. All other clients are able to do this, but this client's downloaded CSV' s always reformat the data so she's unable to re-upload them.

Importantly, when the client is instructed to transform the data back to UTF-8 in Excel, the data reformats into the correct layout, but she is still unable to upload it to the network (the CSV is rejected and the error says the format is incorrect).
However, when I take her spreadsheet and transform the data by the exact same process, I'm able to upload it to the network just fine.

Question

Why is my client's CSV data laid out incorrectly when she downloads the spreadsheet?

Context information

• My version of Excel: Office 365 (16.0.14026.20304) 32-bit

• Client's version of Excel: Office 365 (16.0.14026.20202) 64-bit

When CSV files are opened in Excel, columns are derived from the list separator, i.e. a , or a ;.

Which character Excel automatically recognizes as the column separator is dependent on the regional settings of the system.

To maximize compatibility try saving your CSV (with proper columns) as an Excel file (.xlsx or .xls) and send that to your client. Those filetypes do not depend on the list separator character.