If you're out of data, don't stream video.
If you're out of data, don't download big files.
If you're out of data, don't use websites with a lot of data (such as autoplaying videos), without a plugin or something to minimize it.
If your chat can break your entire 400 k connection just by sending you a link, you should probably look at using some other software. Although of course that's hard, if your friend aren't willing to follow.