As well as the above, Ex 'd' P is being a manipulative git. What he SHOULD be saying is the same as what mummy says. Is the whole notion that it was mummy who did the 'taking away' stemming from him as well ?
"Daddy did something that made mummy very sad. You can stay over at your Daddy's every week but not on a school night as you need to be up early. If you'd like to talk to Daddy more why don't you ask him to ring you more often/every night?"
Your SIL needs to deal with her dd in a calm, reassurng way (and keep her natural upset away so dd can't see). Sounds like she's doing this tbh.
Posting in behalf of my sil, who has recently become a lp. Her xdp had been seeing ow for sometime, and when sil found out she left xdp. Xdp remains in the family home, whilst sil has found lovely home for her and dd (5yrs). Xdp sees dd once a week overnight, the rest of the week he phones when he remembers.
Anyhow backstory over.... Last night xdp rang to speak to dd. dd told xdp that she was allowed to stay that night at xdp. As convo was in speaker phone sil reminded both that as it was a school night and dd ready for bed (6.50ish at this time) it wasn't possible for dd to go to her dads. Xdp kept saying its up to mummy.
When the conversation was finished dd said to her mum 'you took daddy away from me'. Sil naturally very upset by this remark, but didn't make Any fuss of it last night. If dd says this again what you recommend saying in response to dd?