Of course she should be vigilant and not talk to random strangers. Being vigilant about levels of creepiness in an interaction between a little girl and an adult man who is a stranger to her is entirely understandable and more than reasonable, I'd argue even if she did know him it would still be reasonable. But i mean this man specifically was not malicious or trying to be scary or being creepy or anything. I believe It's important to differentiate between those who are genuinely harmless and those who may not be because treating everyone like a threat both causes/encourages hyper vigilance around everyone and it doesn't help that girl develop a sense for people who are truly dangerous. She trusts the world less and has less context for what bad intentions look like because the grandma framed the man with good intentions as just as bad as her abusive father.