Why is everybody assuming that the OP is a Flying Blue member?
This is a first-time poster (welcome, Linnar!) who originally posted this to Travelbuzz, so FB numbers and lounge access are unlikely to come into it.
As for skipping the return... people do it all the time. We do hear the occasional story of an airline coming after someone, but I suspect those are probably people who do it on a regular basis.
However, as others suggested, it may be wise to use something other than a credit card to book your flight if you can.