i realize this "discovery" has really shocked and surprised you. remember you don't have to do anything about it RIGHT NOW, it's not a fire, it's a problem.
now one thing this former crack addict was thinking.....who leaves lines??? who leaves ANY??? especially uncovered under a desk....just doesn't sound like someone super "experienced and proficient" with it all.....
obviously any drug use is upsetting, but perhaps it's not AS BAD right now. talking to your mom about this can be tricky.....chances are good she will try to deny that you see what you see, claim it's left over from New Years 1999, or just get hostile and tell you to mind your own business. but maybe she'll hear her daughter's concern and fear. i hope that is the case.