I'm from France and I've been educated with "eating manners" I consider "normal" (chew with your mouth closed, don't put your elbows on the table, wait until everyone is served before starting to eat...).
I'm OK with people that don't share theses "rules" though.
Thing is, my girlfriend's mother (their parents come 3/4 times per month) has the habit of eating with her mouth wide open. It's noisy, not very elegant (in my opinion) and that makes me slightly uncomfortable, but I'm okay dealing with it. She is very nice and generous otherwise, and I don't want her to be offended -- which she can easily be.
However, I'm afraid that this behavior might cause her trouble in other circumstances, and I think she might be unaware of her way of eating with mouth open: she told us a story about how she was eating a croissant in a café with a colleague, and she dipped her croissant in her coffee. The colleague responded to her in a rude way, saying stuff like "it's truly a shame to eat that way, I can't believe it, if I were you I'd be covered in shame...".
She keeps telling that story like a joke because the colleague would be the one who's wrong because dipping your croissant in the coffee is not that shameful, and in my opinion pretty common where we live. However, I'm seriously thinking that said colleague was rude because of the way she chews...
Now I'm worried that this issue might cause her more trouble: in her job, she will be more and more solicited to business meals or cocktails with people I'd qualify "classy".
How can I tell someone that, in my opinion, eating with your mouth open might offend people you are eating with, and that I'm worried that this could disservice them?
Since this is like a "stepmother-stepson" relationship, we're not as close as family or friends, but still close, so I really don't want to be rude.
I already talked about this with my girlfriend, she's indeed at a better place to tell her. However, she is more reluctant than me to take the risk of being somehow offensive, and cares less about possible ways that this could, again, disservice her.