Sorry to reiterate a previous point I made, but I think he said he'd take her to her mum. No other witness mentioned Libby asking for her mum. It would be relatively easy to manipulate Libby this way. She was freezing, had wandered around distressed for quite some time, she had bloody knees and was in a terrible state. If he told her he'd take her to her mum, she'd be easily convinced I think, because that's what she wanted to do.
Additionally, PR is a fairly young man. I don't know the ages of the other men who tried to help, but I think she'd interact with someone nearer her own age. She'd probably reached the point of total despair. If someone said the right things, she'd go with them because she was at rock bottom.