Is a mother sinful for conversing and joking with her daughter's fiancé, considering him like her own son?

The suitor is considered a stranger to the prospective bride's mother as long as the marriage contract has not been concluded. She must treat him as a stranger, speaking with him only out of necessity, while adhering to the etiquette of Islam. Joking and excessive laughter between them are not permissible due to the general prohibition in Allah's Almighty words: "Then do not be soft in speech, lest he in whose heart is disease should covet, but speak with appropriate speech." She must repent for this, as he is a stranger to her until the marriage contract with her daughter is finalized.