Some people can be asymptomatic. They carry the virus but don't show any symptoms, e.g. some people with herpes are asymptomatic.

Yes, it's still transferable even if she doesn't show any symptoms.
Herpes is very easily contracted. Skin contact and you'll get it already. Condom is not 100% effective against herpes. The person could also be asymptomatic as described above and might not even know that he/she has it in their body. Herpes is also affected by our body immune system, some people are stronger, some weaker. So unless there was an obvious outbreak in his partner and there was skin contact (or sharing of certain things like utensils, towels, toothpaste, etc), it's gonna be hard to pin point how and when did he contracted the disease.