This is a grey zone, even for AF codeshares! If the lounge is run by the marketing carrier, chances are good you will get access. If the lounge is run by the operating carrier, chances are quite low. Another factor is whether the operating airline is a FB partner, for which there might be special provisions (see FB partners info on FB website). In the end, not even FB knows the full answer to your question because it also depends on the individual codeshare agreements. As I said: grey area.