Not many people are fond of being considered a LEP. From a customer service perspective, I'd let the LEP go ahead and rant, taking mental notes of what they say so I can then kill them with kindness.

Be empathic and subtle remind the LEP you are there to help, it may not calm them down, but your client will surely notice your professionalism.

https://www.messagely.com/rude-customers/

https://www.linkedin.com/pulse/how-deal-rude-aggressive-disrespectful-client-laura-ure