Is it really rust or deposits on the dipstick? I would wipe it off with a rag sprayed with WD40 and see if there is any sign of pitting on the dipstick itself. I bet there isn’t. To me it looks like combustion deposits & water vapor.
Does your brother do a lot of short trips, the kind that do not let the engine get up to operating temp? Conventional oil or synthetic? How many miles between oil changes?
I would do a quick search on Toyota and engine sludge since they have had an issue when using conventional oil + longer change interval. I’m not saying this is what he has, it’s just something to be aware of.