'My friend believes that the virus cannot be transmitted when an outbreak is not occurring and therefore disclosure is not necessary, but I believe that the infected person can still be contagious even when an outbreak is not present and therefore disclosure is necessary.'

Transmission can still occur even when an outbreak of sores is not present. This is, as you said, and as many papers say, asymptomatic transmission. Simply searching via the Cochrane library, PubMed, or Google Scholar (Of course, if you have access, medical databases such as Medline can provide further information/a wider range of papers, I've studied medicine and now study biomedical sciences hence I bring up such databases) for 'asymptomatic transmission of HSV' will result in finding articles supporting this. See here as an example: http://jid.oxfordjournals.org/content/198/8/1098.full

Thus your friend is wrong, you are correct.

And in the case of any STD, please do always inform your partner, however embarrassed you may feel. I feel like this isn't a 'German issue', more just common sense/decency/hygiene.