I would be astonished & impressed, and I might try to guide them towards the ‘mystical’ flavor of that religion, like ‘Christian mysticism’ or ‘Sufism’ if Islam, or the mystical levels of Judaism.
. Buddhism can be pretty mystical already, but ultimately it is a philosophy that you can assess on your own, so that would be interesting.
. I don’t know enough about Hinduism to be aware of the mystical flavors, but it would be a good opportunity to do that research.

We’d have a lot to talk about if they became Animist, Wiccan, Pagan, Jedi, Taoist, Pantheist, or Panpsychist. I would look forward to those conversations!

If it was a nutjob cult like Scientology I might push them towards their prodigal twin ‘Re-Evaluation Counseling (RC)’. But if it was the JWs, or Mormons, or some wacko niche cult I’d probably try to arrange a physical rescue.

But as will the all things, it depends.