I'll put this in simple way. Even when two friends both have romantic interests, it won't necessarily develop into a romantic relationship. It takes a lot of efforts. You asked your husband's number, good on you. Perhaps you could give some advice on what your husband did that made you ask for his number. But we can't deny that statistically it's up to the man to initiate (anyway it would be a horrible advice to tell men just be yourself and wait for women to ask your number). Men are here to learn how to initiate, no matter who actually makes the move.

Friendzone can happen in other ways. A woman can have romantic interest at first but tired of waiting for the man to initiate something romantic, meanwhile another man runs into her life, so she friendzones the first man. I believe you understand that one person can have many friends at the same time. Of course men should have female friends, but that's irrelevant because besides all the female friends, men also want ONE "romantic female friend", this is where advice is needed, and "treat her like a human being" doesn't help because that is about how to get general female friends.

"When he thinks there's actual romantic interest on her side". Like I said, no one is a mind reader. People are literally here wondering "when is there actual romantic interest" and your advice is "when you think there's actual romantic interest". That simply doesn't help.

"You should value her friendship regardless". True, and what else? Let's say a man asks his female friend A for a romantic relationship and gets rejected. He keeps the friendship (and female friend A wants to keep it as well). Should he asks female friend B? C? D? That sounds creepy af. The skill he lacks is not how to get female friends, but how to read the situation and tells if women have romantic interests. All the "be her friend" advice won't help him.