If u have not found the answer is because you haven't asked the correct question.
First to understand: DLNA implementations are not uniform, unfortunately. As you found out, certain brand will play this, but same file will not play on other brand(s).
Now the manual: Will say this TV will play AVI, MPG etc blah-blah, but this is not sufficient information. An AVI video file, is just a CONTAINER as we geeks say, you have to know what the video file is ENCODED WITH. This is referred to as what CODEC (look up meaning is needed) is being used. Unfortunately, this information is not published, you the user have to do the legwork.
Steps to Proceed: You, yes you have to find out what CODEC are compatible both for your TV and your friends' TV, if sharing is you ultimate goal. THEN you will have to re-encode the files in question, there are many free video converters, but you will have to spend time converting
them. Video conversion slow.
Tip #1: You have to find out whether an unplayable file has a VIDEO CODEC or an AUDIO CODEC problem. My Sammy TV tells me, I don't know about your friends.