If your SIL spent her time researching the place- it has a good reputation, staff are well qualified & experienced, warm, happy & welcoming, if children & adults generally seem happy & busy (with each other) on arrival, and there is a variety of fun child-focused things to do and If she has a key person that she is happy to go to (and looks happy to see her- genuine warm greeting etc...) Then I would trust that, but keep an open mind about the child's concerns. As it could be related to a specific person, part of the day or equipment (e.g I know a child who hated our nursery because he couldn't use the tap in the bathroom independently)
If this is a new routine & the first time that she has been separated from her Mum, there is potential for a bit of attention seeking here. Especially if you SIL reacts in a very dramatic way when she tells her.
It's tough for kids to get used to new routines, especially if it means sharing with others for the first time, having to learn expectations (and be told no some times!)
I would advise your SIL to try and observe her child playing (without her child seeing her) and ask if the staff have any photos or videos of her child playing with the things that she enjoys (that she can take home)
Use these as a prompt to talk about what the child does enjoy.
Always listen & respond to the child's concerns, calmly acknowledge her feelings "You didn't enjoy nursery today?" So that she knows she can always tell her Mum anything, and try & figure out what aspect of it is it that upsets her, just try not to make the response to her concerns super emotional/dramatic. Also talk to her key person to get their perspective.
Good luck!