you should be able run ddclient on her mac, which takes care of the DDNS stuff. I think it's just a perl script that you can set up to run as a service. I'm pretty sure I used on my raspberry pi, and maybe on my mac mini at one point.

dyndns has their own list of update clients, I think I used to run this on my mac mini when I needed it: https://help.dyn.com/update-clients/

-David