As long as the interest reported on the tax return filing is as much or more than all 1099-Int filed, you are fine.

The reporting requirements for who should issue a 1099-Int is based on who is required to use that form. This client seems to have sold something to someone and that sale and that buyer seem to meet those requirements.

It's not a problem, unless someone has not been properly reporting the info on their tax return. The 1099-XXX forms are Informational. They don't control what taxpayers should be reporting on their returns; they simply provide evidence of some of these activities.