Hi MCR43,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
I am afraid that you can not use your All-in-One computer as a second monitor because the display ports available on the All-in-one computers mostly are display out ports not display in. As a result, they cannot be used as monitors.
However, check to see if your AiO computer does have the HDMI in port.
Please let me know if you need further assistance.
Have a nice day.