In that case, definitely speak to school. The fact he’s Disney Dadding it means he is going to be blinkered to possible threats and the danger there is that something will have to happen before he pays attention and by that point, your DD will have been exposed to something that she shouldn’t have been.
What sort of activity is it is it mainly blokes? I wouldn’t be keen on a 9yr old posting online to any mainly adult community, but if it’s mainly blokes online then absolutely not. Has your daughter read the comments?
Have a talk with her about online safety (if you don’t know about tech stuff, don’t worry about not knowing and start finding out about it. If they’re playing games, get them yourself so you know how they work. Things like Minecraft and Roblox have chat options and they’re effectively playing with the whole world if their accounts aren’t set up correctly). No joining people on Discord etc, no friends unless you know who the other person is and have met them in person and most importantly, let her know that whatever anyone may say to her online, no-one can hurt her or her family etc (threatening to kill family members is a great way to scare children into doing things). I never give mine into trouble if they’ve done something they shouldn’t ie friended someone. I just block people and remind DC that not everyone online is who they say they are.
It may sound melodramatic, but you’ve got to know about these things. You have to know where the danger lies so you can stop it happening. Chances are it will never happen, but you lock your door at night to stop someone robbing you during the night.
Slightly different situation, but we had a guy that used to live in our town visit from time to time. He had an interest in local history etc and was welcomed by a few folk, especially one family who happened to have children under 10. Turns out he was a convicted child abusing ex-copper. Someone warned the family and showed them evidence of his conviction and time served in prison. They still were reluctant to condemn him. He’d groomed the parents. So, your DH saying he’ll monitor things, he’s already starting from a weak position as he’s in the “All this danger round every corner nonsense” place. Paedophiles will know the signs. They know who doesn’t have good protections. He’s got his 9 year old daughter online in a mainly adult community.
How interested in it is she? Is she likely to get bored of it anytime soon?