Step daughter and ex girlfriend dilemma add in a break out of head lice...and it makes one big problem..ugh!?

So last weekend my step daughter went home after my fiance's weekend with her, I have 4 kids of my own btw...Well come Monday morning her mother messages me angry and saying that every time her daughter comes home from MY house she's infested with head lice. For one I was offended that this was being brought up to me by her and not my fiance apparently he was embarrassed to bring it to my attention. She went on to say she wasn't going to allow her daughter over anymore because of this, and then started rambling on trying to get me to feel sorry for her because she's a single mother "doing it on her own". Basically the message seemed to be a wolf in sheep's clothing. My biggest concern here is what should I do now, I have FOUR children to treat for head lice and most parent's know this takes more than one treatment and allot of hard work. My fiance still wants to bring his daughter over but not have her spend the night and "make sure we send her home clean". My concern is also for MY children, she can't say this started with my kids and if she does come over the ex could still SAY her daughter went home "infested" again just out of spite. Does anyone have any kind and helpful advice??