Someone above already mentioned this, but it does seem to be more about the dynamic between these two particular children. With my, now 4.5yo, there are certain kids he still has trouble with on playdates. It's mostly ok now, but there are definite combinations that are more prone to conflict.
A couple of thoughts:
1. If this is a tight-knit community, is it possible to have some ground rules in community areas? That way, any adult can guide their kiddos (and other kiddos) under the same general rules. (i.e., no hitting/shoving/pushing/etc. AND these are community toys and will be shared.)
2. I would really work on empowering your dd to assert herself in these situations. I think this is done best with modeling it for her initially. I.e., "This is dd's toy and she would like it back now. Let's find something else for you" or whatever. Simply asserting your dd's desires will empower her and teach her how to assert herself in those situations. And, IME, it's absolutely fine to take the toy from the boy if he is unable to give it to her himself...this can be done gently.
3. In the case of pushing her off of the tricycle, I would be very firm that we do not hurt one another. If his mother didn't step up to do this, I'd have no problem doing it myself. I'm a firm believer in the "it takes a village" philosophy and fortuntately my friends are on board with that. If, indeed, these ride on toys are for the community then I would have the boy hop off the trike and ask your dd for a turn when she's done.
One other thought...I have a feeling he was much more interested in the reaction that was caused by this toy in his hands rather than really playing with it. I'm totally assuming, but based on the OP's presentation of this situation it sounds more like the dynamic between these two rather than the specific incident. It sounds like this child needs boundaries and limits and also needs help understanding how his actions affect others. From the sounds of it, his mama is doing him a disservice by her lack of response to his aggressive behaviors.
What a tough spot to be in! I would try to focus on it more from a community perspective. I would step in with other children when boundaries are being crossed. I think you can model that for this other mama...she might appreciate it. I get the feeling she might have NO idea what to do in these situations...she might just welcome your input and help with it.