No, it's not anywhere near 100%.
For this, I'm going to assume she has genital herpes type 2. It's the most common to have genitally, but she could also have type 1 genitally if she got it from someone performing oral sex on her. I'm going to use type 2 though because it's been studied, and it's more infectious. If she has genital herpes type 1, assume the numbers are lower.
So we know that herpes sheds. This means the virus is active on the skin, but there are no symptoms. It is infectious as this time, but we don't know how much virus it takes to transmit, or how much virus is present from person to person. It can also vary from situation to situation on how much virus it would take for one person to get infected vs another.
Shedding rates: (and you can find all this in the herpes handbook - https://westoverheights.com/herpes/the-updated-herpes-handbook/)
HSV 2 genital 15-30% of days evaluated
HSV 1 genital 3-5% of days evaluated
HSV 1 oral 25% of days evaluated
HSV 2 oral 1% of days evaluated
So assuming your gf has ghsv2, these are the transmission rates:
Ghsv2 transmission, female to male, over the course of a year, assuming sex 2-3 times a week:
Only avoiding sex during an outbreak - 4-5%
Adding condoms OR daily suppression - 2-3%
Adding condoms AND daily suppression - 1-2%
So if all you did was avoid sex during an outbreak, the chances of you getting hsv2 from her are about 1 in 1000 sexual episodes.
Is she on suppression? If she isn't, that might be something you all discuss.
If every time the condom breaks, are you going to be panicked? I understand that no one wants herpes, but since you are in a relationship with someone who has it, you have to figure out if you are able to live with the risk. Some people are, some aren't - that's okay. I'm not judging you. It's just a lot more fair to you both if that you can't handle it, you let her find someone who can, and you find someone without herpes. If you realize you can't, let her go gently and respectfully.