When it comes to tricky situations, hating your friend's date (or, worse, their significant other) is right up there.
We're not talking about the sort of dislike which comes from thinking their jokes are lame or not agreeing with their politics, though that can be frustrating, too. We're talking about when you are certain this relationship isn't doing them any good but, for whatever reason, they seem to be oblivious to it.
What do you do? Is there anything you can do?
"My first piece of advice is don't be the judgy friend," psychologist and dating expert Melanie Schilling told The Huffington Post Australia.
"This can often be the hard part, as you have all these thoughts and feelings and only want to protect them from a doomed relationship.
"However, the moment you step into judgment, you shift the dynamic in the friendship. Once you start coming from a place of judgment rather than love, it can damage the trust in your relationship.
"So I would advise against launching into 'here are ten reasons your partner is wrong for you' or anything like that. It's not going to help things."
Instead, Schilling advises asking questions which will help steer your friend to their own conclusions.
"What you can do as a loving friend is put on your coaches hat. Ask your friend the right questions to help them arrive at their own insight -- help them to explore it on their own. By doing that, you can facilitate their own self awareness.
"Getting really future focused is a good one. Questions like, 'how would you see your life five years down the track?' Get them to describe how they would you see their partner fit into that picture. Also ask what their partner is doing today that fits in that future picture."
Of course, things become more complicated the more serious the relationship gets, and approaching the situation even more fraught with complications.
"When it gets to this stage, it's particularly hard, because of a thing called sunk costs," Schilling said.
"The idea is because they’ve invested in this relationship -- time, emotional energy, finances if they've moved in together, but mostly their time -- it's really hard to walk away.
"It's a psychological thing, when you have put in all that energy, you feel like you need a reward. If we don’t have it, we are going to hang around until we get it. It's irrational but it’s not uncommon."
Schilling says it's also common for the person in the relationship to have 'positivity bias', meaning they tend to maximise the positive aspects while minimising the negative.
"Essentially what that means is the relationship not going well but they have a really strong hope that it will work out," Schilling said.
"Their partner might be totally emotionally unavailable -- they might prioritise their sport or put their career over their partner -- but if they give their partner the pleasure of their presence for one dinner a week, they will react with 'oh they're so great, we have dinner together every week.'
"Basically they have given them crumbs but they are going to hold onto that and treat it as proof they are loved."
If you do decide to have the conversation with your friend, Schilling says be prepared to be met with a high level of defensiveness.
"That is not about you as a friend, it’s about where they are at with their level of awareness about what is going on," Schilling told HuffPost Australia.
"There is a very high possibility they will defend their relationship, especially if they have moved in with the person.
"On an unconscious level of course, he or she doesn’t want the embarrassment of having made that mistake.
"They've had their blinkers on, have been going through the motions, and now have moved in together. They are probably desperately hoping they can get this relationship back on the right track and are minimising the negatives while maximising the positives.
"The stakes are higher because the relationship is more serious. It's important you are very gentle with them and encourage them to arrive at the conclusion to be arrived at on their own."
Failing that, Schilling says unfortunately there's not much else you can do.
"You will have to let them make their own mistakes. It's really, really hard, but he or she is an adult and they get to make their own decisions about that stuff.
"I know it's frustrating and you just want to scream at them. But the best thing and the most emotionally mature thing is to do everything you can to help them arrive at their own conclusion."
However, and this is important to note, Schilling says all of this goes out the window if your friend is in a relationship that's dangerous to their physical or psychological health.
"All this advice flies out the window if they are at psychical or psychological risk," Schilling said. "There is a big difference between an abusive relationship as opposed to them not being right for each other.
"In the case of the former, all bets are off."
If you or anyone you know is experiencing domestic and family violence call the national sexual assault, domestic violence counselling service on 1800 RESPECT (737732) 24-hours a day. If you are in immediate danger please call 000.