How to delete incredibly long text thread?

[zettasyntax]

Hello, I've seen a few posts about the stock messaging app starting to really lag when it reaches a large number of messages. My mother has a Note 4 and she never likes to delete anything, so her thread with me somehow has managed to reach 7,900 texts and it goes so slow. I've tried deleting the thread from within the stock messaging app, but the phone never manages to get it done. I've tried deleting the thread from Hangouts and Google Messenger, but the thread didn't from the stock app. My mom isn't really the type that could pick up a third party app (I had enough trouble teaching her how to use the stock app), so switching to a third party app isn't really an option (even though it does get rid of the lag). I saw another post on here from about a year ago that mentioned a third party app that was able to get rid of a pesky long thread in the stock messaging app, but they never mentioned what the name of that third party app was. Does anybody know of an alternate texting app that will be able to delete the thread in the stock texting app as well?

Thank you very much for any help. It's really been bugging me. I had tried backing up the messages and restoring them, but the thread with over 7,000 messages always goes slow, so I've realized that it must be the fact that there's just way too many texts in that one.

 

[anon(632115)]

OK just to make sure you're pointed in the right direction we need to know what version of Android is running.

 

[zettasyntax]

The phone is currently running Lollipop 5.1.1

 

[anon(632115)]

OK Open up messages, click on your mum's name,click more, click delete and select all messages (in the orange box) and hit delete

 

[zettasyntax]

I've tried to delete them that way and it never seems to work. It says "deleting" for about 10 minutes and the messaging app just freezes, so I just shut off the phone and tried again and again. I can delete individual messages, but if I delete more than a few at a time, the app just crashes. I was really wondering if there was any alternate texting app that would allow me to delete the thread so that my mom could just start fresh thread with me in the stock app.