Your client often struggles for words. This is very obvious when you quickly switch a topic or ask them to complete a sentence.
They may also take a while when asked to name something quickly. It may seem like they don't know the word but in fact they just have trouble saying it quickly. But their speech is otherwise fluent and they don't have generally slow reactions.
It looks like they have trouble remembering but in fact it just takes them longer to get the word out of their memory even if they remember it.
It is not clear what exactly causes this issue. Some people struggle for words because of general mental impairments but also many people with very high IQs do.
There is much disagreement among researchers about what it means but it is clear that it very often accompanies dyslexia. In fact, performance on tasks involving rapid naming predicts relatively well whether someone will later struggle with reading.
There is nothing in particular you need to do. Just be patient and do not judge the person you're dealing with this.