Though I don't have experience specifically with this happening after license expiration, I have had to deal with orphaned MDM devices which I would assume would occur once the Systems Manager license expires. Assuming that's what would occur:
1. Will the settings remain on the smartphones that he has already setup?
Answer: Yes. To remove the settings, you will need to either unenroll them first via Systems Manager, unenroll them on each device (if you allowed to be removed from the device), or erase each device AFTER the license has expired.
2. will the installed apps remain on smartphones?
Answer: Yes, unless it is an app set to remove with MDM and you or the client were able to remove the MDM profile from the device.
3. Is there anything that he has to take care of that may happen if licenses are not renewed?
Answer: No, if you unenroll the devices BEFORE the expiration period. Otherwise, they will need to remove the MDM profile on each device or erase it.
If I were you, I would start unenrolling them a couple of weeks before the expiration to give enough time to take care of any devices that may be turned off/lost network. If they are tied to something like Apple Business Manager, remove those as well. And if I were your client, I would erase the devices anyways.
