Speaking from experience and from general knowledge, women typically are more territorial about places like kitchen and bathroom and decoration of a place etc Men tend to be more easy going about these things. Typically grown women (not college age girls living in dorms temporarily while in college) don't want to share their kitchens with other women or other women decorating their living room or have make up in their bathroom etc You want to build a household with your man, not with your man and another woman.

I know you said your lack of experience doesn't matter. But it does. You only lived with your parents. You never lived with other people, people you aren't related to. Not only you'll be living with a man the first time, but there will be another woman living there. You might not even envision how it's going to work out.

She isn't in need though imho. She might not be able to have a certain life style but it's not a need. People with below average income still find places. Maybe not as good as your place though. It's a "want", not a need.

On the other hand it sounds that helping this woman is a great concern for you and your SO and you believe she is in real need. Maybe it's wise for him to move her in and you stay with your parents because you have a place to live and she doesn't. Eventually she'll hopefully move out. I know it sounds kind of ridiculous. But so is your SO's proposal.