I would refer to the recent FTC questions to “algorithmic pricing practices”. Long story short it was about some companies are using browser data, accounts data, etc. for “smart” pricing. Your brother may not care about someone watching him but I don’t think he wants to pay for the same goods more than others.
Problem of Proton Drive is not only in the missing linux client. There is no support of WebDAV that makes integration with many open source tools (like note taking apps or photo management tools) almost impossible. Nextcloud is much better for me. Without a WebDAV (or one of other popular APIs) it looks like an attempt to create “walled garden” with vendor-lock on Proton, imo.