Wow, I must be lucky. I have never had any issues with it and my iPad and Mac are both 8 yrs old. The iPhone is the only current Apple hardware I have. I will try a hard reset on the iPhone thanx.
The first couple years AirDrop was very unreliable for me. Rarely worked. I got so accustomed to that even though I use it all the time I'm still surprised that it works. Now it works great for me.
Some other things you can try on
both devices to narrow down the issue, reseting covers it all though. You can start with hers though.
Turn personal hotspot off (on both regardless).
Airdrop settings and set it to "Everyone" see if it pops up then set it back to "Contacts only" and see if its still there.
Turn off connected bluetooth devices
Toggle wifi off for 10 seconds and back on
Toggle bluetooth off for 10 seconds and back on
Make sure low power mode isn't on
Reset iPhone.
Airdrop uses bluetooth to make a secure wifi connection between the devices.