They should have checked before booking. Then decided if they wanted the flight. For anything not 'bog standard' you need to check. But by all accounts (see past threads here in SLF) and experience of colleagues, an unadvertised service of KLM is the high probability of losing your bags when you transit through Schipol. I work on the basis that generally speaking, if airline A offers better value for money - which doesn't necessarily mean alower price - than airline B, then A gets my business. But it depends on how one judges 'value'.
Having said which, a typical 'where you are screwed' is a BA codeshare transatlantic where for timing reasons, you have to take the codeshare operated by American Airlines. Who are pretty crap on service these days, even in first class.