People here in the comments say it's a heel-turn for the nice guy. I don't buy that interpretation.
The context is that it's a guest exhibition within the professor's exhibition and the "nice guy" was already picked as the photographer to guest. There were a few photos that the MC made in that session. Here the professor sings praises to one of those photos and with the phrasing "you should have tried this earlier"... GUT-PUNCH CRITICISM.
The professor doesn't realize this of course, but the "nice guy" clearly heard it: "You suck. You're a failure. You have skill, but no talent." The eyes go empty, the response is automatic, the mind goes blank. This is not a heel-turn - this is the realization. Remember he originally mentioned that "someone helped him with the photos that he would like to introduce to the professor". Now the professor doesn't need to remember this and in this very moment when the "nice guy" gets slammed with the most painful of criticisms - he also blanks out.
From this the likely outcome is that the "nice guy" will start feeling resentful towards the MC and potentially would grow distant and cold, BUT I don't think he will do the standard heel-turn of suddenly going cartoon-villain style and start actively sabotaging her. This manga goes quite deep into issues of disenfranchisement and self-esteem, so we will have another interesting emotional hurdle to jump over for the MC - AND for the "nice guy" too - cause he'll have to accept his photography skills being on sidekick level compared to the MC's talent and that is not something easy to accept.