I would not tell the mother- it sounds like you are making a judgment regarding her decision to have the older child in daycare. Honestly, she is probably exhausted and tyring to get used to the "up-all-night" again and needs her rest. Most of my friends have sent their older children to daycare after the birth of a sibling (sometimes just 1/2 days or 3 days a week). If their children expressed anything to the provider, they never knew.
I'm sure that she does want to be home b/c she has no idea how "quiet and boring" home is right now! I would encourage her to talk about the new baby, bring in pictures, build-up the "big girl going to school" concept and let mom rest.
If my provider said something to me, I would be really upset and probably keep her home... which would mean less sleep, more crankiness, less good time together, etc. Those of us who work full-time have a hard time when we are "full time" parents- I know, who admits to that? Well- me. I pay my daycare providers to enrich my child, love him and play with him when I can't (for whatever reason).
It won't come off as "not wanting to watch your daughter"... it will likely come off as "you aren't spending enough time with your child", which isn't the message you want to send to someone who is a hormonal roller coaster and who is completely guilt-ridden over working and having her kids away from her all day.