Agree with Loukr. If the chap were managing, say, a college shop, and was making passes at students in this way, he would be disciplined, maybe sacked.
I don't know about a 'duty of care', but he does have a responsibility towards all his staff to maintain a safe working environment and, I would imagine, an atmosphere conducive to teamwork. Whether his attentions towards the girls are welcomed by them or not, the team dynamic must surely be hindered by it. If you are concerned, then other members of staff must surely be aware of what's happening.
I'm sure if you contact his managers, they'll treat the matter discretely. Better than him being had up for assault by one of the girls or her parents. Tesco not acting on this contact and then something serious happening could result in a lot of bad publicity for them, which they can hardly afford right now.