Download old version of Messenger
Would like to give my old iPhone 4 iOS 7.1.2 to my mother-in-law and wondering if she will be able to download old version of Messenger after I swipe the phone and move it under her apple ID?
iPhone 4, iOS 7.1.2
If you're talking about Facebook Messenger, I would ask directly the manufactorer's support team if the app is still available for iPhone 4.
Please read this support guide before you give away your device:
What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support
Regards
Giulio