So just looked into it due to this post (the app is kind of fussy, so I usually only use it to read comics I'm already subbed to xD;; ) but:
1.) I don't believe the icon is any help, but if you tap a series to open up it's "table of contents/summary", you can look up by the subs/views/likes statistics. Premium comics have an additional statistic, a rating, denoted by a star. If you see that star icon, the comic is premium. Otherwise it's free.
2.) The search bar doesn't seem to have any advanced options as far as I can tell, but on the app if you scroll alllllll the way to the bottom of the main page, there's a link titled "Community Comics". If you follow that it takes you to a page that filters out all of the premium ones. Further there is an "Explore by category" (or worded similarly) that you can use to further sort the comics by genre. Thankfully for your friend, BL is its own separate category from Romance, so you can direct them to just follow these instructions ^ to find comics to read :>
3.) tbh the search bar should have some options tho lol