My son (6m) found his dog dead at their dads (36m) house

I’m (34f) pretty furious and I just want to make sure I’m not overreacting.

My husband and I separated a couple years ago. He took our boy dog and I kept the girl in the split.

I asked for the boy dog back on multiple occasions (he was kept in his crate far too often IMO, then his girlfriend kicked him outside when she got a cat; I wanted him to be home where he was loved; I was denied each time).

Well.. he died the other day. My son (10m)called to tell me. Apparently my 6 year old walked out to him dead with flies buzzing around him after their dad picked them up for his weekend.

The girlfriend was home the whole time and he was just dead in the backyard. They claim he got into something and got sick. But… died immediately? This was a previously healthy 3 year old lab mix.

The call: “hi mommy. doggy got into something he shouldn’t have. He got really sick, and then he went to heaven”.

My gut is suspecting foul play. And my heart aches for my 6 year old who witnessed that and is now traumatized.

I don’t understand what happened or how. But I do have him home now. Albeit a little too late.

How do I bring up the conversation of what actually happened with our dog? If she did something to him I definitely don’t want my kids around her. Do you think he suspects her? He was grieving pretty heavily, so I doubt he was involved.

Thanks.