1)UK citizens dont need a visa perse coming into canada as a tourist
1b) electronic travel authorization form is not required for UK citizens coming to canada over land border.
2)she was staying at Workaway, which depending on how you interpret/misinterpret is "working" (which it possibly is, a little shady on what they are.) so canada may have thought she was going to work... which a tourist visa doesnt cover
so my guess is canadian authorities felt she was coming here to work, which she didnt have the proper paperwork so got denied.
USA authorities upon re-entry attempt, probably felt she is scamming the ESTA 90 days being on a "4 month" trip, staying at workaway locations... and playing devils advocate, there is no proof that this is NOT what she was trying to do... going to a short trip in canada before going back thinking it resets ESTA (they have to be gone from usa for a reasonable time)
so much unknowns.
i think they were right to detain her/deny her entry... but the length of detainment is at issue.
very first thing i think of when viewing their site is this is some lodging for volunteer/unpaid labour. https://www.workaway.info/