If it's a software issue then you could restore the iphone. Restoring the device SHOULD clear up any issues if it is software related. If that doesn't work I would take it into an apple store. Let them take a look at it.
Not sure if this has anything to do with it, but Apple recommends that you drain the battery at least once a month and charge to a full 100%. Keeps the battery calibration correct I've been told.
I admit that I am useless when it involves an issue with Mobile/Smart Phones. Computers/laptops ; well, I think, I am getting into them.
Although your 'old man's' iPhone is turning off at 45% battery level, is he still able to switch-on and use it further. If so, I would go along with what has been indicated by Jomlett and endeavour to exhaust the battery, prior to a full re-charge.
Thanks guys. When the phone gets to 45% , it then shows 1% battery and goes off. So he isn't able to drain the battery cos it says it's already drained. It's a puzzle!
I would take it into a shop and ask them to replace the battery. I had one that did that, went down to about half-charge gradually, then seemed to be suddenly flat. A new battery sorted it out.