I think this is great advice. I think it is also important to say no to him and for you DD to say to him that she doesn't like some of the things she does (as much as she is able to at her age), but I suspect if you just continuously say no to him without offering any alternative, he will just feel the need to show you that he has power and you can't boss him around which might make him worse. I think if you set up situations where you are able to give him positive attention, he might respond really well.
If he is feeling really deprived for attention, negative attention is better than no attention at all. Some kids would rather do a behaviour and at least get attention in a negative way than to be completely ignored all the time. If his parents only give him attention in a corrective way, his behaviour could be conditioned to doing only things that get him negative attention from adults because he doesn't get any at other times. If you show him that he can have attention from you when he is gentle or just because - like sitting down and having a friendly chat with him about nothing in particular - his behaviour might change dramatically. Sometimes kids like this just want a bit of attention and direction ... not always to be told not to do something, but to be shown how to play. It is possible that his parents have never taught him how to play gently with other kids. You could be doing him a huge favour.
It does suck that your DD is getting hurt, especially when he is so much bigger. I hope you can work out a solution to keep everyone safe :hug:

