Is this something to be concerned about? My friend's son is hurting my son.?

I have a friend with two sons, ages 5 and 3. I usually catch up with this friend once a week, usually at her house, and I bring my two youngest to play as well. They are 3 and 18 months.

My friend s 5-year-old is apparently going through a stage where he doesn t like babies. OK, whatever. But it has come to a point now where he deliberately tries to hurt my 18-month-old son. For example, when we went over today, I took my kids to play out in the backyard cubby house. My friend s 5-year-old starts hollering at the top of his lungs (by the way, his speech is very unclear and he really needs speech therapy), runs inside and grabs his Lego gun that his mum told me he made "for shooting babies", and starts pointing it at my 18 month old, saying "bang, bang, bang". Mind you, my son doesn t take his toys or do anything like that, he moves around and explores and does his own 18 month old thing.

After that, the 5 year old started ramming the gun into my son s shoe until I told him to stop. then, when my son was on the swing, the 5 year old came over with a hand-held metal rake and deliberately scraped my son s leg with it. I picked my son up and told my friend that if he does anything else, I will have to leave. my friend finally decided the discipline her son by locking him inside so he couldn t hurt my son any more, who was playing outside. Prior to that, there was no action taken, no telling him to stop, no time outs, no apologies, no explaining why we don t do that, nothing. I don t think the kid has ever been disciplined. He is starting kindergarten in January (Australia). Do you think this is an issue, the violence I mean? I know it doesn t seem like much now, but deliberately following and hurting someone who is smaller than you, someone who is innocent, for no reason ... would you see that as something to worry about. The 5 year old has seen a psychologist in the past for other things, should his mum mention this behaviour as well? I am just worried and I don t feel safe leaving our kids to play when we visit.