I would be asking for disposables at least until mom gets the diapers under control at home. It probably means she is laundering them wrong and a build up on them has made them less absorbant. They could be not fitting well anymore including not coming high enough at the waist, tight enough at the legs so may need to simply purchase some new ones. Elastic can only stand so many washings and wearings before it stretches out.

At minimum if she continues with the cloth she must send child with waterproof plastic pants over the diaper to contain the excess. I assume you are not rinsing and simply bagging all diapers, clothes, etc to be sent home. I also would not bathe the child. I would wipe his down as best I could - old rags from cleaning bin. Then call the parent to pick up and take him home. After a couple times in a week having to take child home she may agree to disposables on her own.