Don't send the email. Based on my experience, I predict that the anonymous email you are proposing to send almost certainly won't tell the speaker any information she does not already know, only something that she is either in denial about or that she is (for whatever mysterious reasons of human psychology) helpless or unwilling to do anything about. On the other hand, especially due to its anonymous nature, the email is quite likely to come across as hurtful and to cause her negative feelings such as guilt, self-loathing, low self-esteem, depression, etc., that would make her situation worse without leading to any progress towards resolving her accent/language problem.
There is a time and a place to offer people negative feedback that might help them improve, e.g., when such feedback is directly solicited from you or when you are a person whose job it is to offer such feedback. That time and place is not your current situation. So don't.