He's not a "consenting adult" in general and abstract, he's a diplomat and must maintain a respectful image. He can't do whatever he wants without it being the business of anyone else, because he's a diplomat, a public employee. If he wants to be free to party, resign from a public job and its accountability.
I know two similar cases:
Two Peruvian congressmen that were caught partying at a nightclub in Brazil and are now waiting punishment by the Peruvian Congress ethics comission:
http://journalperu.com/docs/Velasquez_Vega_SaoPaulo.pdf
While not recent, the Brazilian ambassador to Cuba which was a renowned partying host and guest that was sacked last year when his fame became unbearable for the government.