“ Tarry this night, and it shall be in the morning, that if he will perform to you the part of a kinsman, well; let him do the kinsman's part; but if he will not do the part of a kinsman to you, then I will do the part of a kinsman to you, as YHWH lives. Lie down until the morning.”
Boaz then told her to wait there with him until the morning when he would discover whether the nearer kinsman would be willing to fulfil his duty as near kinsman. If he would, then well and good. (As with most ancient marriages love did not enter into it). On the other hand if he would not perform the duties of a kinsman, then Boaz himself would take up the role. ‘As YHWH lives' was a regular oath, and a binding one. He wanted Ruth to be in no doubt about his good intentions. The request to remain the night was no doubt for Ruth's own safety. That he saw nothing inappropriate in her doing so comes out in that he could have suggested that she found a place to sleep nearby.