Been through this a few times.
Make sure they are put into lost mode in iCloud. This will keep the tracking working either through wifi/your sim or the same thing that makes itags work depending on the models.
Once they have come to terms that they will not get them back, while they are still in lost mode set them to erase.
Once erased the location will disappear in find my.
It is important that they do not remove the device from their apple accounts it must stay there.
Once erased the thieving scum will need to sign into the icloud account to make use of the phone if it is kept on the apple account. By removing it you are giving them a free phone.
The client and their medical id contacts if setup will receive icloud phising smses, whatsapps etc that will try get their account details and/or unlock code. Tell them to never follow any links that they get. Apple will never contact them.
Depending on the network they used you may be able to see the IMEIs of the devices if a SIM the client owns was in them.