Whether a user receives an email notification for a share (or anything else) can be configured by the user himself on his personal page. So not the "sharer" decides if the user receives an email, but the user himself - which is actually a good thing
If the user doesn't want to receive emails in general for shares there is probably not much you can do. Another option I see is to share via email. When you enter the user name in that text field, it usually offers the email address of that user as well. Clicking this, will send an email with an individual public link to that mail address. Side-effect is of course, that this is a public share. If someone gets to know the URL, he can do everything what you allowed the user behind that email to do.
Maybe mentioning the user in the comments of the shared file/ folder is something applicable for you. The user will then see a notification upright in the corner, next to his user name/ avatar.