Lower back pain between medial (spine) and lateral sides… not spine pain though.

Patient: My partner has been complaining of sore hips for a week and its worse after sleeping on his side. The pain is just above his hip bones and half way between his spine and his side. Ive felt it and it feels like theirs a spine like bony structure above his hips but on his back. And to my knowledge and checking my anatomy books theres no bones there. Could it be “knots” in the muscle? It seems to only be on one side or could it be more serious? Its definitely below the kidneys but just above the hips.

Symptoms: Pain.