Not necessarily. I live in a dwelling built entirely with 2X 3's.Standard doors are designed for 3 1/5" studs with sheet rock on the inside and sheathing on the outside, so roughly 4 1/2" jambs. You can order doors with wider 6 1/2" jambs. I can't tell what is going on with your door jamb from your picture. You don't want to include any of the trim in the measurement. The brickmold trim attached to the outside of the door isn't included in the jamb thickness
What is the construction of your house? Is it a 2x4 framed house with siding, 2x4 with a standard brick veneer, or solid masonry? From your picture it looks like you're including the storm door frame in your measurement. Is that the case? You want to measure just the door jamb not including any of the trim. So you measurement should include where the deadbolt and latch go into the jamb.
I've previously asked her for panoramic photos.
From the photo in #10 were she to place the tape at where the 1-1/4" mark and measure to the 7-3/4" mark, she should have an accurate jamb width measurement.
Installing a prehung door is fairly DIY friendly if you're handy. I'd want a helper, though a single door I could do alone. I installed a double patio door on my basement project and it would have been extremely hard to do by myself. This may be a time when you want to bring in an experienced friend to help or hire it out if you've never done one.
However, there can be flashing issues, based upon the exterior coverings, which are beyond the DIY knowledge level.