Does "女朋友" work for older couples?
Using archaic phrases like 红颜知己 or 忘年之交 is a bad suggestion, these phrases come straight from dated Chinese movies. No modern human talk like this in real life. It is like saying " She is my father's fair lady" or " she is my dad's dear confidant." in English.

If they are very close, saying she is 女朋友 (girl friend) is perfectly fine in today's open minded society. 

The safest option is to call her your dad's 朋友 (friend). or 好朋友 (good friend) if you are not sure they are officially a couple or not.