I don't have a source for empty tubes, though I know other artists on this forum have used them, Michael Georges being one of them.
As far as tubing pigment with medium in them, any harm that could be done would depend on what's in the medium your friend is making. I'm VERY careful about what I will mix with my paints and haven't found anything more trustworthy than linseed oil.
You might want to check out some of the threads on the use of different mediums in order to read about concerns that others have regarding the impact of experimental or unknown substances on the longevity of paintings.
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