Hi Jesper - I had a daughter who was adopted and her and her adoptive father found me as follows:
1. They got her adoption certificate which gave my maiden name
2. They looked for a marriage for me which they found which gave my father's names, so they got my birth certificate to prove they had the right person.
3. They looked for any children of the marriage and bought the birth certificate of the youngest which gave them an address. They wrote to me at that address but, unfortunately, we'd moved, but the people who lived there gave them some clues as to where we'd moved to. As we'd moved again that was a dead end.
4. After that it was basically just looking through electoral rolls in the library for my husband and I, there were too many as we have fairly common names, so they then looked for one of my sons who has a less common name. There were only 4. They looked them up in the 'phone book with the intention of ringing them all. However, the first one they rang was my son. They just told him they were doing family history research and wanted to contact me. My son gave them my address and 'phone number, and my daughter wrote to me.
That was about 8 years ago now. I've since met my adopted daughter and her adoptive parents quite a few times.
It's always worth a go.
Lizzie
ps. I know it's suggested that adopted children apply for their adoption records and go via social services and maybe that would be a better thing for your friend to do so that social services can contact his birth mother in case he is a secret. My adopted daughter was a secret so I didn't tell my husband for a few months, fortunately his response was "Let's go and meet her".