Help with ad hoc app

I have an ad hoc app developed for my client a year ago.  It has been installed on an ipad air 2013 (not ipad air2) The ipad needs to be replaced, and my client is asking for recommendations for a new one. Unfortunately, my developer died suddenly, and I have not located someone to offer advice to me.  I THINK I recall him telling me that a system change could render the app useless.  I do not remember if it was ios, hardware or both. I'm guessing ios, but with hardware updates since 2013, I'm not sure.  I don't want to hamstring my client with an old device, if they can safely upgrade. (The app will only be used for another year, if I'm not mistaken) But I also don't want them to have to rebuild (other than adding the device id) for the remaining time.  Is there anything I need to know in order to recommend a new device to my client?