When OP don't help with the sharing situation (toddlers)

[Mummabear04]

DS is going to turn 3 years old over the summer. He isn't at nursery yet but I do take him to toddler groups and he has a 5 year old sister so has plenty of time with other kids including play dates etc. He's generally OK with sharing but the problem is when he's around other boys his age he will want to play with vehicles and his favourite thing to do is to park them in a line. Obviously if there are enough toys then this is fine and usually at toddler group it works OK because I can help him to take turns and he knows the other boys so trusts them to also take turns. Today I was at a different group and another mum/son combo were there that I don't know. There was a plethora of vehicles so thought it would be ok for DS to line and few up and park them but this little boy just kept taking the toys he was playing with and then following him around every time he moved doing the same thing. DS has an ear infection at the moment so isn't on his best form and started howling and getting upset. The OP just said "oh sorry it's the last thing they learn" whereas inside I was feeling just tell your son to wait his turn instead of letting him take the toys. DS obviously is learning too and tbh after a good few sleepless night my brain just wasn't working. I took DS to a quiet space and calmed him down. When we came back in DS tried very hard and then eventually asked for a turn of a particular car. This other boy then just continued to follow him around, trying to take the car back and bothering him with the OP doing nothing to help. I'm just wondering how OP handle thing kind of thing? Obviously my DS needs more practise sharing as he is still only 2 (his sister will just give him anything he wants and he's still got the learning curve of nursery) but I just found it so hard that the OP wasn't doing anything to help the situation meanwhile DS is quite clearly struggling. Would it be unreasonable to say no to the little boy if his mum wasn't stepping in?

I don't think there's anything wrong with saying something like "DS is having his turn now. He will give you the toy in X mins when he's finished".
That's turn taking. If they aren't at the stage where they are playing together then that really is as close to sharing as you get.

I think it's important to advocate for your child, whilst also putting in place some boundaries ie. He cant have all the toys to play with an not let anyone else play with them.

When the child puts the toy down, pick it up and say “DS is playing with this one, you can have this one instead” and hand them one your DS is not playing with. Stand physically in the way of the boy if he is following your child around. If the child was almost at the stage of bullying my child I would tell them directly why it’s not on and completely ignore the fact the other parent is there. They aren’t being parented at that moment so you have to do something to protect your child.