My dd did the same thing with her first playdate at around the same age! It was soo embarassing because the mom was there and her son was being bullied by my dd. Everytime he tried to hold a toy she grabbed it and said MINE, even if it was one she never touched for weeks.
Some ideas that have helped us.
-- maturity. She's now 29 months and understands about sharing a little more (doesn't always do it though)
-- have that mom bring some of the child's toys to share with your child.
-- put your child's favorite toys away
-- model how to trade one toy for another. My dd will now offer an alternate toy to the child and say "here, take this" and often the child will drop the desired toy and take the one my dd is offering. I was SO PROUD of her when she did this instead of grabbing, and proud of myself for modeling it.
--explain about taking turns. Say, "It's John's turn to play with the toy right now, you can have it in five minutes." Tell John (or whatever his name is) that he has to let your child take a turn, and try to give a 1 min. warning, then trade toys.
-- if your child grabs from John's hands, grab it back and give it back to John and then try to teach your child to ask nicely and trade something. If John still won't give it then try to distract with another toy or even a treat.
-- do whatever you do as the consequence when your child hits or spits or whatever when you are alone with him. With my dd it's an immediate time out in our room behind the baby gate. She knows that when we have guests over she can get away with more but lately I've been doing a time out even if it makes me look like a strict parent, because otherwise the behavior doesn't stop. I also talk to her about how it hurts her friend and he's sad and gets scared of her when she does that. Once she screamed at her friend's face several times and he started to cry. I held him and she got jealous but I ignored her and comforted the boy (I was babysitting him). She saw that the consequence to scaring her friend was that he got all the attention so that stopped the behavior. Once she forgot and started to scream, then said, "No screaming. Spencer got scared."
-- give a pep talk about sharing and stuff before John comes over. This helps soo much with my dd. Before we go on a playdate I'll say, "Okay Abi we are going to Spencer's house. You have to share with Spencer, okay? Be nice to Spencer. Ask nicely to share toys. No hitting Spencer, no spitting at Spencer." And I do this 2-3 times on the way over, or before he comes to our house. After the playdate I praise her and talk to her about how nicely she shared, and how she didn't spit and I'm so proud of her and how Mommy's very happy, etc. It really does work, even if you think he's too young to understand, he does.
7yo: "Mom,I know which man is on a quarter and which on is on a nickel. They both have ponytails, but one man has a collar and the other man is naked. The naked man was our first president."