...spam, and potentially malicious. The link was to some weight-loss site. She immediately shut off the computer, and called me for help (I'm a programmer, so I must know what to do, right?).

Well, this might have just been spam with no malicious payload. Or, it could have something bad along with it. I told her to get Malwarebytes and run that, but could she already be infected with ransomware, and just turning on the computer normally might lock her up? What is the the procedure to attempt a clean in this case? I don't know if I'm being overly cautious or not.

She's on a Windows machine, and I think it was merely a URL she clicked on, and not an attachment.