Maybe if any have leaves big and smooth enough to wipe off you can try washing them with a damp cloth.

If I were you, I'd probably try wiping the leaves with some white vinegar and then rinse them off with plain water in the shower/bath tub. (Don't get any vinegar on the soil since it might be harmful to plants that don't like acidic soil).

This post was edited by summersunshine on Sat, Jan 31, 15 at 18:13

If you know what kind of plants they are, the most specific advice could be offered. Some kinds of plants wouldn't mind having their leaves removed if they are in good health, and conditions they find acceptable for re-growth. Most kinds of plants could be given a shower, though there could be some variables you'd want to check first, per individual plant.

The plants are a Wandering Jew & a Philodendron

Wandering Jew is very hard to kill and I've had multiple off shoots come out of just bare stems I've stuck in moist soil. I bet it would be fine if you were to cut off its leaves.
My wandering Jew started from a small 4'' cutting on the floor I found after I stepped on it at homedepot one day..... And now I have multiple plants and offsets and I cut back and throw away large quantities of it every year but it still is going strong.

For the philos however I doubt they'd come back as easily as the wandering Jew would..... I'd try the vinegar method and put it in the shower.
Good luck with them

Maybe try removing the old soil and putting the plants in new soil and in a new planter....maybe cigarette ashes have fallen in the soil if the person ever smoke around it....just a thought....that's what I would do along with cleaning the leaves like others have said.