At one time there was a Java application called ForUML that could read modern Fortran source (i.e., OO F03, F08?) and generate UML diagrams. Last I looked, the official URL for it had gone dead. Someone captured some/all of the software on github, but it hasn’t been maintained so has suffered bit-rot and no longer works (out-of-the-box, at least for me).
Here’s a recent publication from the original author. There he indicates it is available “upon request”. Earlier it had been freely downloadable.
If anyone has more up-to-date info about ForUML please chime in; I found it to be a useful tool.