Forgiving grammar gaffes is difficult. I’ve found that if it is a recurring problem, then offering positive feedback is an effective way to make the writer aware of the issue and, hopefully, encourage him/her to get help.
I’ve also noticed that many “grammar errors” are just stylistic differences. For example, the Harvard comma! I have also found that, in some situations, a document will go out for review and, depending on the reviewer, will come back with completely different solutions to grammar “errors.” I have also known of people who have used previously-reviewed templates and been chastised for grammar errors that the reviewer had previously inserted!