Ex has roomate sharing room with my kids???

My ex husbands new wife decided to move her friend, girl in mid 20s down to stay with them . I knew she was staying there but i find out from my girls that she has been sleeping in thier room in one of thier beds. My kids have 1 room with two twin size beds. Now they have to share one twin bed and this chick is right next to them. That doesnt seem right to me. I asked my ex to move that girl in another room just on the nights that my girls are there and he refuses. My kids are upset and say that they feel uncomfortable with her int he room with them sleeping. WHAT CAN I DO???? Please help they are supposed to go tomorrow night for the weekend!!!