No, absolutely not. Social Security benefits may not be garnished to satisfy a civil judgment by this type of debt collector. Further, it very well may be a violation of the Fair Debt Collection Practices Act (FDCPA) for the debt collector to threaten garnishment when the law clearly provides otherwise.
I think it would be wise to put the collection agent on notice that your friend will not be bullied. Here is a sample letter that your friend can use to assert his rights under the FDCPA: http://cli.gs/nocollect.
One final thought: your friend's Social Security check cannot be garnished, nor can that money be touched even after it is deposited into a bank account. However, other assets, including money from other sources in a bank account remain subject to seizure by a judgment creditor. It is always a good idea to segregate Social Security money from other funds or assets.