I think it would have been reasonable for Mrs Jennings to give Marianne a few days to a week to weep over her lost love. Then, if I were Mrs J and Marianne weren't better, I would send her home to her mother. I agree that she can't be expected to coddle Marianne forever.

But the point about Mrs J here is that she's treating Marianne, as the text says like a child on the last day of its holiday, rather than a young lady with a broken heart. She thinks that one bout of crying and a party that same evening will 'fix' everything, perhaps because she's irrepressibly jolly herself and it's likely Charlotte and Lady Middleton inherited that trait. She doesn't understand Marianne's temperament at all.