Always check a professional’s credentials before leaving your pokémon with them. Anyone can create a seemingly legitimate business these days, even if they actually lack the credentials or training to perform the skills they’re offering. It’s especially a problem in regions where criminal organizations (namely Team Rocket) are active, as many will use these businesses as fronts for money laundering, if they aren’t simply attempting to scam customers out of money or their pokémon partners.
So yes, verify a business’s credentials first. Read reviews from other customers. Do a lot of research on the business and, if possible, the person running it. Never assume that a business is legitimate, just because it looks that way at first glance.
(Also, don’t feel bad about falling for this “singing coach.” There have been reports in Kanto and Johto of a pair of Rocket underlings scamming the same trainers out of their pokémon using entirely different scams every week for years. Some con artists are simply good at what they do.)