As George says, foreign pension and housing allowance (as with other benefits-in-kind) are taxable.  The golden rule that all income is taxable except specifically exempted still applies.  Housing allowances spent on qualified housing expenses, on the other hand, may be excluded under §911.  In addition, statutory pension contributions often are not characterized as compensation.

Under some tax treaties, however, certain types of pension contributions may be tax deferred but could still be subject to limits that others apply to qualified US plans.  You will need to refer to the relevant clause(s) in the treaty of the country in which your client is a tax resident for the details.

Depending on the type of investments within the pension plan, your client may also have exposure to PFIC and other information return filing requirements (including FBAR and FATCA).