Friend pregnant with twins 1 disappears ?

So my best friend was supposed to be having twin girls. She told me that one of the babies hearts stopped beating at 4 months pregnant. The other twin is alright. I asked her what will happen when she gives birth and she said her doctor said he doesn’t see the other baby on the ultrasound. The way she explained it to me was basically the baby sort of disappeared and this is really hard for me to understand. Does anyone know anything about that happening? Im really confused and hurt 😔