The link in the follow-up email is good for 3 months. That timeframe is probably related to the drop of sessions from the Past Training tab and several reports after three months.
If they lose the link? This is a concern. The post about saving a certificate link from the session and substituting any attendee email in the URL works, as long as it is within the three month window. Myself, I just keep an alternative certificate format and type in the person's name.
Also of concern is that the attendee can not save a digital copy of the certificate, unless they do a screen capture. The attendee may assume that they can save the file and print it later. No. The file format of the certificate is svg, which does not save the specifics (name, class, organizer, date). The svg format only saves the vector drawing of the certificate.