well, its hard to say who should & shouldn't trip, but if your friend is severely depressed it may be a bad idea. However, these days SSRIs get prescribed to almost anyone who walks into a GP's office & mentions being sad, so he may well be fine to trip, psychologically. Tripping on SSRI's (i was on them for a while) is certainly blunted. Same goes for MDMA. I'm not sure why serotonin syndrome was mentioned, as i'm unaware of any case of it stemming from use of a psychedelic or entactogen while taking SSRIs. Now, MAOI's, slightly older antidepressants, are a serotonin syndrome risk. Prozac is an SSRI though.
If he's tripped before, both on SSRI's & off, i'd venture a guess that you'll be ok, as long as you keep the dose reasonable. And if you got the 2c-e recently, DEFINITELY DO A LOW DOSE TEST (<1mg) before taking a normal dose, as some bromoDFLY (dose of less than a milligram) was recently sold as 2c-e, and no one's really sure how much of it was sold.