Can I dump a girl after finding out she has an std?
Oh my, why do you want to dump a girl after finding out she has an std?
Because she got it from sleeping with someone else.
You must be feeling hurt after finding out your girlfriend has an STD.  There is nothing wrong with dumping her for cheating on you, and even more so for having unsafe sex in the process.
There is nothing wrong with just not wanting the disease either.
You can end a relationship for any reason and it is understandable that you don't want to be with her anymore. She betrayed you by cheating on you and almost gave you an std.