This is Harassment. The most inappropriate and totally unacceptable type of behavior in a corporate culture. You cannot ignore this email, this behavior has no place in any company.
What to do?
Most large corporations have an anonymous line to report such incidents. It is usually a website, so that you do not need to use your own name or email.
If her company does not have that, HR is the place to go. Some companies have a board of business ethics, that would be another option. (I met such a board when I married my customer, they were independent and pragmatic.)
Why do it?
Because this senior person is a threat to the company, its people, its customers and its reputation. It is not only your friend who was insulted, it is also the company that has been compromised.
UPDATE
Ok, so based on the comments, anonymity needs to be clarified here. An anonymous line gives you the opportunity to submit your incident without your or anybody's name. Just like you submitted here on this forum. And you can ask for advice. The good thing is that advice in this case comes from within your own company, own culture from people who are designated to handle such sensitive matters.
Can you stay anonymous? Well, if they advice you to submit the person's name so that they can reach out and take some action, you still have the option to evaluate the advice and decide if you wanna proceed or not.
If you proceed you'll have to share the person's name with someone on the help line. They will keep your anonymity, the person who wrote the email however will probably guess that it was you. (unless he sent several such emails to many people). The help line can explain better how this works within your own company.
The point I want to make is, that there is often a professional and anonymous help line who can guide you how to manage this situation, with real anonymity at least int he beginning.