bro, u need to read up and experiment with exposure.
1. F1.8 is too wide open for board day light! use F8 if directly under the sun, unless u are under a shade/roof, then F1.8 might do (but i advise F2.8) ur shutter speed cant compasate the over exposure at the max speed (1/2000, or 1/4000)
2. flash is auto mode, when they detect u have too much sunlight from the WHOLE picture, the flash will fire at minimum powweerrr, so u need to do some read up and play around with manual flash

3. with diffusal.... =.= WRONGGGGG. it totally use for indoor to stop ugly shadow, and when u uses iso400 and above. but u can use iso400 at boarddaylight!

4. how to get bokeh but not over expose, for wide open lenses like F1.4-F2.8 for daylight shooting, ur shutter cant move faster then 1/2000 for entry level, or even for mid level, 1/4000 might not be fast enough! what u need is a +2 ND filter to cut down the light going into ur lens! (but warning! ur flash might not be strong enough to light up after using ND filter!)
ermmm. i think my advise arent really giving the direct answer, but all ur setting is wrong

perhaps he was using spot with the subject near the center....i would try to use matrix with flash under day light..