If this is an accurate measurement, I'd probably go with something more like 26x1.6.

I hate to question you again but that's a pretty wide rim. I've got a couple of MTB wheel sets in my garage, and their outside width ranges from about 25mm (the stock wheels on my daughter's old RockHopper) to 22.5mm (the Salsa Delgados on my Muirwoods 29er). Inside width is usually about 5 mm narrower. A relatively new rim, designed for the 26x2.35 and wider tires that are now common would be as wide as you describe, but the 26x1.95 tire you mentioned seems to point in a different direction.

Obviously, a tape measure isn't the best tool for this job and a measurement in millimeters is ideal, but (also obviously) we work with what we have.

You may be OK with 1.25, but at 15 mph, I don't think she'd suffer with wider tires. Check out the Michelin Country Rock. They're dirt cheap and get great reviews.