In terms of getting SE into a car, his warrant card really can't be underestimated. It can wield a great deal of power, influence and trust when you're a law abiding citizen.
SE's interaction with LE was probably minimal up to this point and positive. She's also been widely described by friends, family and colleagues as a lovely, helpful person. Not naive, just inclined to think well of people with little reason to be suspicious of authority.
Sadly, I can envisage multiple approaches by WC that would have been effective.
Negative approach: LE says you've done something wrong. You worry that you have and defer to them.
Positive approach: LE says they need assistance. You switch into good citizen mode and try to help.
Positive approach: LE offers assistance. You switch into polite mode and accept.
edit: also, it's possible he was in uniform. He isn't supposed to be off duty but well ... that doesn't mean he wasn't. If WC wanted to take advantage of the extra authority it afforded him, he may well have been.