Two points: 1. "I know she's somewhat of an over protective mom, but I am not able to watch her child in that same way and she should not expect that."
But you *did* lead her to expect it. In fact, you agreed to it. Why? To me, that's more puzzling than her original request. I would never have agreed to a level of supervision if I wasn't willing to honor it. Ridiculous or not, it may likely be the case that this girl might not be able to go with your daughter in the future because of your actions--or she's in the position of NOT telling her mom, because she doesn't want to get both herself and you in trouble for something she then had no control over.
That mom might have had other reasons to have her daughter supervised (such as previous shoplifting) which she did not make known to you out of discretion for her kid. She is not obligated to share that with you, IMO, if it was previously resolved. No parent wants to tell another parent "Oh, we had some trouble with sticky fingers a year or so ago..." IF you agreed to that level of supervision, she wasn't obligated to expand on her reasons.
2. THAT said, I think the level of supervision at the mall would really have to depend on the level of maturity of the kids and how prepared they are to deal with unwanted situations. I now know 14 year old girls who are regularly hit on by older guys these days, invited to go back to their cars, out of the mall, etc.. We didn't really have that problem back in the 80s, where I grew up. I went to the mall with siblings starting at 13(I was the eldest or second-eldest on these trips) and our mom just dropped us off with about $5 each for lunch. NO Problems! Never had to call mom. However, in my later teens, I had a couple friends who shoplifted on mall trips. I was forewarned and more or less ditched my friends while they were doing this (best way I knew to handle it at 16 and 17, aside from discouraging the friends). This stuff happens with teens.
I worked at the mall, too, and before that, at a pizza place that was held up at gunpoint. That's neither here nor there.Once again, it's the level of maturity and how each kid conducts themselves....
So, overprotective? Maybe. It really depends on each parent's perspectives and experience.
But no matter how justified or weird I feel another parent's request is, if I'm not up front with that parent, there is no way that the validation of others that the request was strange would lessen my breach of trust with that other parent. Sorry.
(if it was true, too, that she had heard her son had been robbed, can you understand why that would resonate with her? no, you might not be able to stop something like this from happening, but you would be present to help the girls emotionally afterward-- maybe this was what she was hoping for?)