How do I say no nicely?

My friend asked to stay with my husband and I today. She has to leave her current apt soon and doesn't want to stay with family. We've had our issues lately, but her and my husband already don't get along. I don't feel comfortable with her staying here. And it'd be an indeterminate amount of time until she gets back on her feet. What's a nice way to say no?
How do I say no nicely?
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