You’ve actually got more options in terms of apps and approaches than you might think, but one app in particular does the job very well and for free: TeamViewer QuickSupport. Not only can you see for yourself what’s happening, you can also try and fix the problem yourself remotely, over the web.
The answer is to get the person you’re helping to share their device’s screen with you. Just trying to work out exactly what is (and isn’t) happening on screen is a challenge in itself, and that’s before you even get to the stage of thinking about what the underlying causes might be. Troubleshooting issues over an audio call or via text message is typically an exercise in frustration.
It’s a problem you may be familiar with, especially if you’re the designated “tech support” contact for your family, office, or group of friends: Someone gets in touch wanting help with a problem on their phone, but you’re not actually with them to take a look.