A thread-unsafe DLL can be set to run on the UI thread from the settings of a Call Library Fn Node.
You can read more about the various execution systems in Multitasking in LabVIEW
Even if you use the standard execution system, the user interface is still separated into its own thread. Any activities conducted in the user interface, such as drawing on the front panel, responding to mouse clicks, and so on, take place without interfering with the execution time of the block diagram code. Likewise, executing a long computational routine does not prevent the user interface from responding to mouse clicks or keyboard data entry.