Looking for advice in delicate situation please.
We left our DD at family party for 7yo with plenty of relatives to look out for her. She is used to spending time there and there were lots of children there although DD was one of only 2 girls of at least 10 children.
In asking about the party on her return she mentioned the above incident. I asked whether she told a grown up and who else was there at the time. She said it was just the 2 of them in a part if the garden not visible from house, other children were in other part if garden or in the house at the time and that he had told her not to tell anyone that it happened. I asked why she thinks he did it given that the garden isn't large and there are several toilets in house, she said he just needed to go there and then but she didn't see his boy bits.
The boy in questions is DS of a relative. All wider family have perceived for at least 10yrs that he has some SEN issues that probably need intervention or at least support. He is a v sweet and polite boy but his behaviour is routinely very disruptive, v over excited and unable to play in a calm way or avoid breaking toys or damaging furniture/ornaments esp when he was younger, on one occasion he spoke to his mother in what I would regard as really quite scary aggressive/threatening tones when he realised they wouldn't get home in time to watch his favourite tv programme. Having started secondary school, apparently he has v few friends and has an arrangement to stay indoors at lunchtime as the playground presented to many challenges. From what I know, his mother does not consider that he needs anything other than love and attention in order to overcome earlier disruption in his life (he is not biological DS of my relative but she has had him since he was a toddler). Am not sure where my relative's DH stands on this issue but he clearly seems to get very stressed out by DS' behaviour. Wider family have gently tried to suggest getting additional support for him but response from mother has not been positive. I'm not close enough to know full details of this but have never passed judgement on how my relative is bringing up her DC (her other DC, also not biological do not suffer from same behavioural issues). We've tried to accommodate her DS and e.g. intervened to defuse situation when other adults are getting stressed out at his behaviour if his parents are not around etc.
Anyway, with this recent incident, my DH is absolutely fuming and wants me to address it but I'm not sure how to go about it. I've briefly discussed it with the host of the party who was mortified and v apologetic to us. I assured them it wasn't their fault as the DC are old enough to look after themselves playing in enclosed garden and I wouldn't have spent the whole party following DD around the garden anyway had I been there. The host agrees this is a tricky situation but that it needs to be communicated back to the DS' mother.
AIBU regardless of the outcome of the conversation (when it happens) to avoid DD interacting with this boy or should we try to be part of a wider supportive network to help him?
Any advice gratefully received on this point as well as on how to raise this issue in the first place.