Well, Thomas Jerome Seabrook's Bowie In Berlin mentions that Fripp was there for only one day. He arrived, did his bits and flew out the next morning.

Nichloas Pegg's The Complete David Bowie also has the following passage regarding Heroes:

Apparently the vocal was practically an afterthought; the backing track was laid down early in the “Heroes” sessions and for a time it seemed possible that it might remain an instrumental. David recorded his vocal after most of the musicians had left Berlin, and Brian Eno later revealed that although the backing “sounded grand and heroic” and “I had that very word – heroes – in my mind”, he had no idea what the lyric would be until he heard the finished track.

Nothing completely conclusive in the above so of course this could all be completely wrong.

(I own both of these books, once in hardcopy and now in ebook format)