This means that either you, your partner, or both of you should be clear that no level of intimidating behaviour will be tolerated. It is also strongly advisable to present a united front to a bitter ex-partner.

Because having that united front is so important, some experts believe that if your partner’s ex is harassing you, the first thing to do is to talk to your partner about it so that you are on the same page.

In your joint effort to fend off the harassment, be sure to remain transparent about every interaction. You might even want to keep a journal of these events, should the abuse escalate into something with criminal elements.

On the off chance that your partner is not in agreement with you, or thinks you are being paranoid, you may have to seek professional help from a relationship counsellor.

Valentina Tudose, a relationship expert and certified hypnotherapist, has pointers on how to take a step back, analyse and approach the situation objectively before making a move.

“In such cases, when all attempts of reconciliation fail, there is a strong feeling of rejection and self-judgment that can lead to anger, bitterness and jealousy.”

“Not only does the ‘victim’ feel betrayed when the relationship ends before they are ready, but if the partner who initiated the break-up quickly enters a new relationship, things can get really nasty really quickly,” Tudose warns.

Fortunately, there are ways to avoid this predicament before initiating a break-up.

Tudose says that when couples are willing to admit their relationship is no longer serving them and is causing more harm than good, the bitterness of rejection can be counteracted by having open conversations at an early stage.

“As all conflicts in relationships are caused by needs not being met, it is a (relatively) simple matter of identifying and acknowledging whether the unmet needs offer any room for negotiation, or if they are just unsolvable,” says Tudose.

But if you are watching your partner suffer at the hands of a bitter ex, then what can you do to help them?

Still looking for ‘the perfect partner’? Stop – it’s not going to happen

“Anger, bitterness and the desire for revenge are all indications the person’s expectations are no longer met, so a reframing of those expectations is needed.

“Discussing the new reasonable expectations and giving the person a chance to adjust is a necessary step in the break-up process,” she says. “But if they are unwilling to listen, these new ‘rules of engagement’ can be expressed in emails or letters. And if needed, they can also be legally enforced, should such extreme measures be needed.”

If your partner’s ex does become bitter and vindictive, you need to have patience and be clear about your feelings and boundaries. However, it is vital that your partner – not you – communicates these expectations, as the ex is less likely to respect boundaries set by a stranger; this is especially important if your partner and their ex share children, as it may not be your place to interfere.

If you ever feel that your partner is in danger from their ex and you are aware of past instances of violent behaviour or brushes with the law, then you may need to step in sooner rather than later.

This will inevitably be a challenging field to navigate, but at the end of the day, open communication with your partner is always a good place to start.

Luisa Tam is a Post correspondent who also hosts video tutorials on Cantonese language that are now part of Cathay Pacific’s in-flight entertainment programme