You may need to find the app before figuring out what platform.
If it turns out that what is needed is an iPad, then they could look for used, for a decent price. Every iPad from iPad 2 forward is capable of handling the current operating system release. Prices will depend on how new, and how much storage.
If the platform for the app is going to be something else (most likely Android, there is not so much software like this for Windows tablets) then you have to make sure the particular tablet has the right version of the operating system and adequate resources (storage, working memory, battery life, processing power) for the task. There is a wide variation in price of Android tablets because there is great variation in capabilities. If the job requires the power of a Galaxy Tab, a Kindle Fire is probably not going to work out so well, even though Amazon sells that one really cheap.
Again, you can get a lot more Android tablet if you buy last year's model, with little difference in capability when it is at the same original price level. As soon as the new model comes out, people are trading in or selling the old one, and vendors are trying to close that out.
If the application does not need phone connectivity (i.e. WiFi is enough) that saves money. In Galaxy, that means a WiFi only Tab, rather than a Note. Not having a phone running saves battery life. Turning off WiFi when not needed helps as well.
Most of the (bar) musicians I've known are still using paper, except for piano men with big repertoires, and for them the platform has usually been a MacBook, not cheap. I don't know what application. Even some of those guys are working with a 4-6 inch thick notebook they've been building for twenty years.
Tom Test
Itasca Spirit 29B