The dance graduate played Cecily Cardew in the play, which enjoyed a three-night run and was in aid of the charity Action on Addiction, of which Kate Middleton is patron.
The Daily Mail reports that Harry giggled his way through the comedy, in which Cressida was wearing a flowing white dress. Aww.
According to the Daily Telegraph, a source said, "Harry was clearly there to support Cressie and seemed to want to talk to her afterwards.
"He left, then came back inside for a bit, but eventually left alone."
The former couple dated for two years until April 2014. But everybody (or is that just us?) has been secretly hoping the pair will reunite ever since.
The pair would hardly have time for romance even if they wanted to, as Harry is about to jet off on a trip to Australia for a month, and then will head to New Zealand, before leaving the Army after 10 years.
Here's why Harry's probs still in love with Cressie (well, we like to think so):