Generally, purchase of a burial contract will be allowed. There probably is a limit as to the amount that can be spent. COA will need to be advised.
Consult a local attorney before you take any action, and make that consult sooner rather than later.
Unde PA law, your mother is free to do so according to the following
340.7 BURIAL RESOURCES
Burial resources include both burial spaces and burial reserves.
55 Pa. Code 178.3
The CAO will verify:
Resource Disregard and Exclusion of Burial Plots-OPS031102