Hi Stephan...
Starting with PellesC 7 the default standard is ANSI C11, which has the gets() function dropped.
Instead use the fgets() or gets_s() functions as described in the help file.
If he's not using any other C11 features, he could also set
Project -> Project Options -> Compiler -> C Standard = C-99
Although... as you point out
gets(string) is really just an alias for
fgets(string, stdin) anyway...
To avoid buffer overruns, the best option would be
getline() which allocates it's own memory. (which he has to
free() when he's done with it)