Not a perfect answer but perhaps you could have 2 apps, one for my macros or something and one my fitness pal for example.
There are trackers where you can see your clients intake, but that doesn’t sound like it really fits what you are asking.
Or another solution: use an app that also can be used on their web app or website. You’d have the downloaded app instance logged in for yourself and then create a shortcut icon to the Web app on your app screen with your boyfriend logged in and do it that way. I use avatar nutrition which is a tracker but also a coaching app (it adjusts goals based on your progress), but before ethey had an android app I did exactly this where I shortcut Ted the Web app and it worked like an app so I imagine that could work for some!