Ask them to call their internet provider and to just ask if they are doing any form of maintenance that might be affecting him in a particular way and also have them send a report of it directly to Larian using this form maybe its a known issue with Mac users;
https://larian.com/support/faqs/how-to-submit-a-bug-report_85


They can also try troubleshooting with this guide but I don't know how relevant that would be for a Mac user but maybe enough to locate the crash logs in case its not connection based. https://larian.com/support/faqs/crashes-during-gameplay-pc_61

Hope this is enough help to get somewhere with it.