Your client is not eligible for foreign tax credit, neither is US social security benefits earned income for the purpose of foreign earned income exclusion.  US-Chile income tax treaty is also not a relevant consideration because it has yet to be ratified.

Since US social security benefits are US-source income, it is taxable to the US and does not create any limitation for foreign tax credit.  If the US social security benefits are also subject to tax in Chile, the question is how and the what extent US social security benefits are taxed in Chile.  Chile generally provides for a relief for foreign taxes paid on non-Chilean-source income so long as the requisite requirements are met; his/her tax accountant should be able to provide further guidance in that regard.