No, respectable and well-engineered streamers do not use DSP to "enhance" the sound, any more than a respectable speaker designer would seek to "enhance" the sound beyond producing something that reflects how he or she thinks a good speaker should reproduce music. Adding DSP effects is your choice as an end user, but would be considered poor practice by a high-end streamer manufacturer.
What your studio engineer friend cannot grasp is that there's a tremendous amount of science that goes into sending a digital signal from one place to another and getting it to your speakers in a form that resembles music. I can only assume that he's never bothered to experiment with such things. Even the basic software involved can change the sound, without getting anywhere near added DSP effects. I have several different Raspberry Pi-based streamers. Apart from the different mechanical designs of each streamer, a change in the base operating system employed can change the sound. Roon, Volumio, Gentooplayer, Audiolinux, etc. can all sound different because of the transfer protocols and adjustments they make to the clocking, CPU, etc. Power supplies, internal design and cabling, and so on, can all make a difference.
I recently pulled out a 1980 CD of John Lewis's "The Bridge Game" and was reminded of how far digital recordings themselves have come since then. For the first time in a long time I heard that good ol' blanket of digital fog that afflicted so many early digital recordings, blunting the transients, smearing the instruments, placing a dirty grey plate-glass window between me and the musicians. Presumably the engineers at Philips thought they were doing a wonderful job at the time, but it's a pity that such beautiful music is marred by such a dreary recording.
I'm also reminded of a listening session that took place some years ago. My local audio club was hosting a USB cable shoot-out. No less a designer than David Berning was in attendance. He thought the whole idea was nonsense but after several hours he left shaking his head and muttering things like, "It can't be possible."
Your studio engineer friend is surely familiar with the work of Keith Johnson of Reference Recordings, who has spent decades improving and refining the recording process to produce legendarily beautiful LPs and CDs. He didn't achieve that by shrugging off the differences in equipment as tricks or "enhancements." ;-)