You can get the webmaps contained within this web application by going to the following URL:
https://theirname.maps.arcgis.com/sharing/content/items/appid/data?f=json
From there, you will be presented with some JSON which can be parsed to show the webmaps within. You will want to find the objects that start like this:
{"type":"webmap","webmap":{"id":"<someotherid>"...
from this, you can then use those other ids within your own application.
These other ids will reference a portal item id that can then construct a webmap
in the runtime.
It might be better for your user to create a group in portal and then provide that group id
to you so you they can freely update the webmaps within that group and your app can then
handle those additional groups.
I hope this helps!