Mom,
first thing this 'bully' aka older sibling, needs is a hug. And attention. From his Mom.
My son does this. to his younger brother. since day 2. Every time, it is because I am distracted. When I am with them both, and he makes a move, I gently remind him that he needs to use his words to tell me what he wants, and he needs to use his words to tell his brother what he wants. Then after all the age-related reminders (like his brother can't use his words yet), and doesn't know how not to reach out (which looks identical to him like 'hitting')I hold him in my lap a few moments until something else catches his fancy/attention.
it is hard to tell someone this - because in this society it is taboo to tell someone else how to parent their child.
You can model for your child the way to do this, too.
Simply be a part of the play, as more of a sitting bystander. You can easily reach in to deflect anything truly harsh or dangerous (which is what needs to be done anyway!) and redirect the older 3 year old (who does NOT know better, and won't truly until he learns it - and that could take until age 9 - or LATER, depending how he is taught vs disciplined (and disciplining a child does NOT teach how to go about this stuff the right way. Discipline is more interested and confused with punishment and removing rights until a child essentially has nothing more to motivate them except to get ATTENTION, which is what caused the breach in the first place. AGH!)
Redirect the 3 year old non-family (or family, it works either way) boy to ask what is wrong, does he need something? More than likely, he will NOT be able to articulate it, or anything. It might be like he's 'not there' or 'not listening'. Simply deflect, ask the question again (because it WILL arise!) and do it again, and again. Ask questions and redirect his attention to the toys or to the weather, or to his Mom, or 'do you need a hug from your Mom?' I'm thinking after the 3rd or 4th asking, he'll ask for one.
Remind him when he comes back for more (because he WILL) that he needs to ask his MOM for a HUG and that's the only way he'll get one is to ASK (even if asking is totally from YOU!)
He's only 3. it does start somewhere but really it starts with understanding that he is still a baby, and even if the skill is demonstrated - it does NOT mean he HAS IT DOWN PERFECT FOREVER AMEN! Every growth spurt will knock it down, flatten it, and send it out to the cleaners.
So if you're looking for fast, quick, easy fix by telling Mom to get her act together, you can, but the fast, quick, easy fix is likely no more playdates.
Good luck,
M.