"or is in an abusive relationship or even something less obvious like my friend is scared of marriage and is about to break off a relationship with someone they are in love with because they're scared."

And how do you propose to distinguish the latter from the former?

Your best guide to action is the person's own preference, not what you think they should want for themselves.

That said, if you suspect that someone is rationalizing an objectively poor choice ( and it would take omniscience to know this for sure ) a good indication is because they are telling you this.

"I know I should leave him but......"

"My drinking is my own business. And screw my employers for firing me from a perfectly good job. I didn't need it anyway."

You don't need to project your values onto others when they're telling you that their strategies don't work for them.

All that's necessary is to offer options and to point out when things don't add up.

They want to stay, and they want the abuse to stop, and they know they should leave the abuse, and he says he'll stop but he never does. Unless something changes its not possible to get everything they want, so they've got to choose.

OTOH, when another is doing something you wouldn't choose for them, and they're perfectly fine with it, and there are no demonstrable frustrations, then just let them enjoy that escargot no matter how squicked-out you are by choices arising from another set of tastes.

"You can't really want that for yourself," is at best condescending and at worst abusive.

Dante