Plants are fine, but like anything else they must be in the correct place and be kept in good condition. This means the plants must be trimmed, neatly potted and dusted on a regular basis and there should be no cluttered feeling or plants with spider or dust webs. Plants must also be placed so they do not block the light entering the rooms, and plants should not be in bedrooms.
If this is your friends house, there is probably little you can do if your friend does not use Feng Shui in their home. All you might be able to do (if asked) is to point out how the plants are blocking the light into the house. However this sounds like this is the person's lifestyle and there is very little you will be able to do to make any change. From a Feng Shui view, it sounds like this is creating a stale atmosphere.
Your choices are:
1) Do I want this person as a friend? If so, do nothing.
2) Have they asked you to help them with Feng Shui? If not, say nothing.
3) If the clutter & dust bother you, meet them outside of their home and say nothing or you will lose a friend.
