Friendica is, apparently, compatible with the Mastodon API so it's apps should work but it might not be a great fit and I've heard reports of apps not working.
Friendica has a list here.
I've spoken to on iOS Mastodon app developer who is looking into how their app works with Friendica but if anyone has tested any other apps that work let us know here. One complaint is Friendica's UI/UX isn't great and having more apps could be a big help.
I just logged into my friendica instance via the mastodon iOS app. I am seeing lemmy and mastodon posts. I only started my friendica account this morning, so I have no idea what content looks like from there.
Scrolling, I am actually seeing way more lemmy posts than I am seeing mastodon toots.
You can use Friendica in Mastodon apps, but you only have the functions that Mastodon has (i.e. only a fraction of the Friendica functions) You have full functionality in the web app and perhaps in the two apps that are specifically for Friendica (Raccoon and relatica). It is also best to go through the settings in the web app. There are many options to select which posts are displayed to you and how, etc. It's best to go through this at your leisure on a (not too small) screen / monitor.