According to Merriam Webster, better half means
the person to whom another is married.
I often see people use a word that doesn't mean the same as its literal definition. For instance:
She is my bitch that actually means She is my bestfriend. Or Sweet heart that is usually used by Mom to her kids, but I've also seen this word used by someone to his/her (boy/girl)friend.
In this case, since M-W defines this word that means the same as a spouse, can we use this word to exaggerate the way we call our (girl/boy)friend? An example a guy calls his girlfriend as his better half:
You are my better half. I can't live without you.