Ok so I need advice on what to do or how to handle a situation. Please no negativity. May be a trigger for some. Here I go.

I bought one of those pop up play tents for my 4 year old daughter today. My friend and her son who is one year older (he’s 5) came over for a play date. Both kids are playing with toys with the door open all the way and I can hear them and see them. My leaves to run to the store for some batteries for the kids toys. (Here comes the triggering part)
My 5 month old poops so I go to change her and as I change her I noticed it getting silent and whispering. I finish changing my daughter and go peek around the corner and both of them are in the tent with my daughter sitting on top of him. I kinda yelled what are you doing in here! And my daughter (who is 4) jumps off. His mom gets back and I tell her immediately what I saw and she took her son to question him while I talked to my daughter. By this time my daughter is in tears. I told her that if she told me the truth I wouldn’t be mad. I asked her if she touched his private area and she said no. I asked if he touched hers and she said yes. I asked if he went under her pants and she said no but he told her to sit on him.
I did tell her that absolutely NO ONE is allowed to touch her in her privates ever unless it’s me wiping her bum after she poops. (She doesn’t do the best job) and I told her that if anyone ever touches her privates again she is to come right to me and tell me.

So I know not to leave them alone anymore but what else should I do? This kinda worries me.