Well, first off, I don't PLAY with the kids at the park either. That's THEIR job. I don't play with them - ever!
But, that being said, I DO supervise them and do it well. I have also been known to meet up with a fellow provider at the park and socialize with her too. But then again, I always have eyes on my kids and my dckids are well behaved and know the rules.
If I honestly thought it was a risk to the kids then, yes, I would say something but I would likely say it to the provider FIRST to see if she improves without overstepping my boundaries and going to the parent.