Yes, there IS a fix!
I'll never understand why people reply when they don't actually have an answer. Fact is, your father didn't buy an AT&T phone and it's unlikely he's going to go sell his and buy an AT&T phone, so why introduce such a non-answer??
Now that I'm done ranting...
If you root the phone and install xposed framework, you can block status bar notifications using Gravity Box (>Status Bar Tweaks > Notification Blocker) or there is another module named NotifyClean which will also allow you to remove the SIM card nag.
I am using a Verizon phone on Cricket Wireless, which uses AT&T towers and have personally used the above solution.
By the way, my reception with Cricket is EVERY bit as good as it was on Verizon, even while traveling across country in some sparsely populated towns. Furthermore, my bill dropped from $420 to $200, taxes included! I also have 50 GB of hi-speed dafa (10 for each line) vs a shared 15 GB across all 5 lines with Verizon. There is only one minor disadvantage. With a Verizon phone, you will be limited to 4g speeds (~5 m/s), and will not be able to take advantage of LTE speeds (~20-25 m/s), but really, I'm incredibly doubtful anyone is actually gonna notice a difference on a mobile device.
---------- Post added at 09:21 AM ---------- Previous post was at 09:06 AM ----------
Thank you sir! I told him to get a ATT or Tmobile phone but he was convinced that the verizon S5 would work due to it being advertised as a GSM phone. He was also having an issue not being able to access the internet without WiFi but someone at the ATT store fixed that for him. Now hes not able to receive MMS messages
Great app with copy/paste functionality for fixing APN settings for nearly any carrier.
https://play.google.com/store/apps/details?id=net.thenatureweb.apnsettings