QRLSP applicable to US SS?

I have a client (Canadian Resident) who received a rather large retro Social Security disability payment - does anyone know if this is something to which the (T1198) special calculation can apply? And if so, how to proceed given that it’s unlikely we’ll be able to have the CRA form filled out & signed by anyone at SS - I assume there must be some official/equivalent paperwork that could support the breakdown?

Yes, it does apply