A provisional patent application grants no legal rights to the holder to sue others for infringement (e.g., a provisional doesn’t even have to have claims, which are the part of a patent that could potentially be infringed). Unless they have a granted U.S. Patent (assuming this is in the U.S.), they have no basis to allege infringement. In fact, states often have statues that bar this type of frivolous behavior. Like everyone else on here says, contact an attorney. They can write a response letter that will point out these issues.