What should my sister do if her husband (my BIL) spoils their child?

My sister is super stressed out. Her husband always spoils their 6 year-old daughter and as a result, she's been throwing herself on the floor while having tantrums and sometimes is rude to adults. He gives what she asks for so she can stop crying or screaming. Sometimes he also laughs at things she does, laughing at what's not suppose to be cute nor funny.

He has no parenting style at all and is the polar opposite of the authoritarian parent, the permissive type that does nothing. He has said many times that he doesn't have the character to be one of those strict disciplinarian, that it's not in him. He's more like the funny and soft towards kids type of guy. Ironically he's very strict as a manager at his workplacea and in other areas, just not towards kids, esp his daughter.

What should my sister do in that case? If he won't do his job in setting boundaries and as he stated doesn't like being the strict one?

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His excuse for her behavior is his same phrase of ''that's just a phase, she'll outgrow it as she gets older'' or ''she's just a little kid''.
What should my sister do if her husband (my BIL) spoils their child?
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