Ummm, in my first post on this I said he might just be a nice guy. If thats the case then she needs to step back before she starts making life difficult for him. I had something similar with our (mine and ex-wife) babysitter. She lived round the corner from us, and if I was driving into town or anything she'd always ask for a lift, and if she was sitting for us and my missus was going clubbing (or shagging lol) after the pub i'd come home early for her and we'd have a cuppa. I had no idea what was going on. I didn't realise the innapropriateness of the situation, until she tried it on wth me in my front room.
My wife went ballistic with me for leading her on, and we lost a good babysitter; this was down to me being 'nice'