First thing to find out is - what's your own hsv1 status. Have you ever had a herpes blood test yourself to know what it is? Can you recall ever having a cold sore yourself?
Taking valtrex at the dose you are taking and not starting it until 24 hours or so after you received oral sex is of no use at all except for if you did happen to be unlucky enough to contract hsv1 orally - chances are good you'd have mild symptoms. I actually recommend stopping taking it now since you aren't really doing anything by doing so at this point other than wasting your gf's prescription. 24 hours after possible exposure is actually too late which is why we don't recommend doing anything like this - impossible to take it soon enough and in the doses large enough you'd need.
Your gf has a well established hsv1 oral infection. She also treated it with valtrex ( not sure what dose - hopefully the high cold sore dose of 2 gms 2x/day for 1 day ).
Between treating it and having it for so long as well as you mentioned the skin was healed chances are good that she wasn't actively shedding the virus anyways. If this even is her hsv1 oral infection just being pesky - chances are good it's from the friction of performing oral triggering it to reactivate again.
So it's not a huge reason to worry. Just should you notice any obvious lesions in your genital area - see your doctor for proper treatment - not the guessing on your own that seems to have occured so far. If you need help with your herpes blood test results when you get them just post their numeric values here and we'll gladly help you with them.
grace
She first had a herpetic outbreak 15 years ago, and she happens to have Valtrex sitting around to take for a second outbreak she is just now having? Well, okay. Something about that seems ... strange, but okay. There is no way to know if the sore you are describing on her is herpetic unless it's cultured. It may or may not be due to her herpes.
There haven't been any controlled studies done on the negative partners taking Valtrex for protection. In theory, it should work, but there's just no way to know. Have you been tested for herpes of both types? It's possible you already have herpes, in which case she can't pass it to you again, since you have the antibodies for it in your body.
You both should get type specific herpes blood testing done to see where you're at. If you end up being negative for type 1, you can discuss what precautions you want to take if you want to stay negative, but good luck with that. Most of the population is positive for type 1 already, and sooner or later you will probably have another partner with type 1 as well.
Rather than you taking Valtrex, the main thing is for her to get the lesion cultured to find out if it's herpes and for both of you to get blood testing done. Also, taking Valtrex "after the fact" won't be protective for you at all, even if there is some indication it can work prophylactically, which means you would have had to be taking it before you came into contact with her sore. I'd just forget about doing that until you figure out what each of you has and doesn't have.