The response of continental legal systems against this kind of sexual act is always changing, and decision by courts try to keep the changes in the public perception of these acts into account.
This service is not meant for legal advice, but I would say that, based on the circumstances as you described them, it would be difficult even to conclude that the act qualified as an act of exposure.
On another note, the problem could be - if your friend is a professional - that even just a fine could "flag" him in the UK if this kind of behaviour is considered inappropriate, but, as I said, I think that he would have the actual possibility to challenge any qualification of his behaviour as an act of exposure.
And I would avoid any "noise" on facts that happened months ago and that, apparently, nobody noticed.
Trying to understand if someone noticed, would be - in my view - a big mistake, as it would risk to make the episode surface.