In my case we were told to only say something if we had been asked to e.g. if PF requested to be told 1 DME before a turn that was OK, but not to actively offer information if not requested to do so (e.g. if you see PF is going to end up high on glide slope, don't say anything). I guess it was a similar brief to what captains get at my company during LPCs i.e. don't lead the FO if you think he's doing things wrong unless it's compromising safety (which is obviously NOT the case during a LOFT, where normal CRM applies). Not always easy.