1. Don't be Moreh Heter. Consider using her work device as gezeilah.
2. Assume the possibility that being that it is a work device, it can be monitored. (Note for employers, MDM's are very cheap nowadays and you can even get free options. Really worthwhile for a company to use them to manage their devices.)
3. Setup filtering at the ISP level like JNet, or at least on the Router. Basic simple option would be to use Cleanbrowsing Family Filter and have someone else lock the router settings to prevent changes.

Tell her you heard a shiur about kedusha in the home. You understand she needs open internet for work, but you feel it proper to be password protected from a hashkafa point of view.

I also thought I was covering my tracks and for almost two years I did (while married). Then about 3 months ago I was caught by wife twice. It was very traumatic. I am very tech savy and my wife isn't so I totally thought I had it covered. It caused us lots of pain, stress and tension. My wife saw some of the disgusting and abominable videos and pictures I was seeing and was rightfully repulsed by it. She has told me that she is still haunted sometimes by what she has seen.

The YH tricks us in to thinking that we can cover out tracks and it will be fine:

1. Are we sure we can cover our tracks? What if your wife needs to use the tablet while you are in the middle of "using it"? What if your family member needs to go to the hospital in middle and you don't have time to "clean up"?
2. Even if we will be able to cover our tracks, is that what we want? Do want we want to remain "safely" in that world?

The embarrassment which potentially may occur when requesting a password to put on pails in comparison to her finding something on there that you don't want her to see.

Thanks for putting it out well. 
I think you convinced me to be more scared but not totally scared.