It's actually really easy to do, I have a dyndns account setup so I can just RDP to my dynamic dns name. Just forward port 3389 on your moms router to her PC and enable RDP.
So, if you have a dyndns name of rdp.dyndns.org you can then RDP into her PC either by opening the RDP client or typing this command: mstsc /v:rdp.dyndns.org