To me, the score per se isn't important. It's what causes the score to be where it is.
A low score might be low due to a one-time disaster, or old misery that is now being worked through and dealt with.
OTOH, a low score might be the result of continuing fecklessness, to use an old-fashioned term: continual late payments with no apparent concern or resolve to change things, new collections, etc.
If it's old stuff that reflects history only and is not recurring, I'd be glad to know and wouldn't worry. (Although I'd keep an eye on things, lol.) But if it's new/ current/ ongoing, there'd be all sorts of bells going off. Is this someone who can't or won't deal with the dull but important everyday chores of life? If so, will they be equally unreliable about sharing the electric bill, doing the dishes, walking the dog, and changing a diaper? It's one thing to raise children who are actually children. It's another to live with an adult who acts like a child.
Actually, this is the real purpose of credit scores for lenders, too: flagging people who have had some sort of difficulty managing their debts and/or money and who are therefore riskier to lend to than others might be. Supposedly the lenders would take a more careful look at those with low scores before offering credit, although the reality these days is that they'll just deny and move on.
Well said, if I was in OP's situation, I would NEVER mix my credit with her, YMMV. For instance, I would never co-sign anything for her unless I can pay for it myself if something would happen.