I don't if your mom would like to invest in these, but here something you can consider and I believe that Duo Kickstarter has started the one with tri monitor as well. Allows you to take your three monitor on the go.
And the biggest joke would be? She looking around to see where you stashed it.
EDIT: Also, next time you bring personal items to work, make sure to get it labeled with an engraver. Hard to scratch that off and if they tried to cover it with something, it fairly easy to peel off. Also, if you happen to see her and she leave something behind, just use it and if she ask, let her know that it belong to the Firm.
Well this portable monitor clamps to the laptop reviews look good
Mobile Pixels Duex Pro - Upgraded 2.0 Portable Monitor for Laptops - 12.5" Full HD USB Screen Plug and Play https://www.amazon.com/dp/B07TDDSJ93/ref=cm_sw_r_cp_api_i_onBgFbS4TW8ZH