On a more serious note though, how legal is it to partially cut out the default textures? I think to make sure you should at least color the frames in a solid color instead of doing a simple cut out.
It's kind of a gray area. It's a significant change from the source material, enough of a change that the majority of the work has been altered. On the other hand, there's still a significant part of the original work remaining so... yea... it's a big 'maybe'.
On the other hand, because this pack is not entirely the OPs work (being a obvious derivative of the default textures) the adf.ly link is without a doubt against the forum rules. The OP whould either remove the adf.ly link, or take folmjolk's advice before the post gets reported.
My friend is planing on making a server. Is there any way to detect if someone is using something like this?
Unfortunately, no. These packs aren't even needed anyway as there are several ways to do this in strait vanilla. Besides, all you really see with them is caves anyway. If you're looking for ores it's honestly faster to just go spelunking or branch mining than to use one of these stupid things.