The ST extract suggests that this individual's social conduct falls very close to the line between acceptble vs. unaceptable. Nonetheless, 'but we had such a laugh last night' tells us that he stayed on the 'acceptable' side of that line (something which is consistent with his living 'qute near the posh bit (of the town, presumably)').

It's perhaps helpful to note the differences between the following pseudo-synonyms in Spanish:

https://dle.rae.es/desvergüenza
desvergonzado: Dicho de una persona que habla u obra con desvergüenza. Usado también como sustantivo.
where:
desvergüenza: Insolencia o atrevimiento

https://www.rae.es/diccionario-estudiante/sinvergüenza
sinvergüenza: Dicho de una persona que comete actos ilegales en provecho propio, o que incurre en inmoralidades. Usado también como sustantivo.

Comparison of these definitions suggests that sinvergüenza falls on the 'wrong' side of the line referred to earlier.

The most-appropriate rendering in Spanish will be very dependent on the intended target readership. For example, a Chilean adolescent's diary entry intended to express the idea of the ST would quite likely include the slang term weón:
https://evanquarnstrom.com/2020/08/12/words-to-know-chilean-...
12. Weón / weona
"This is perhaps the most commonly used word in ‘Chilensis’ (Chilean Spanish). It can be a noun, an adjective, or a verb. It can be used as a friendly way to address a friend like ‘dude’ or ‘man’, or an aggressive way to call someone an idiot. Context and tone is everything. As a verb it can mean ‘to mess around’, ‘to joke’, or ‘to complain’. Whole sentences can be formed just using iterations of this word, as I remember one of my college professors doing to get a laugh out of the Chileans while the exchange students didn’t understand a thing. The list goes on, but understanding this word in its hundreds of meanings and forms is key to becoming a true Chilean."

Only confidece 3 because there are too many possible variants, depending on context we don't have, and the target readership.

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Note added at 1 day 3 hrs (2025-05-17 20:28:16 GMT)
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@Juan Jacob
In Chile, 'cabro' is very commonly used in informal contexts to mean 'friend', 'companion', etc., just as in English we use 'kid' to refer informally to male children and adolescents.