What I would do depends on how well I know the person. Whether they are open to my influence or whether they have given me the permission to influence them. That is probably the biggest thing if they have given me the permission to influence them in any way.
First try to realize that a person who complains a lot, who is very negative, is not very happy. Try to wish Compassion for them and develop more patience. To prevent you taking on their negativity or Dukkha, you also need to develop Equanimity, understanding that their thoughts belong to them and you don't have to take them on. So if it's got to the stage where you have to develop Equanimity, then we develop Equanimity by realizing that their thoughts belong to them and go back to your work.
Sometimes people who are like this, don't like to listen to people who are cheerful or try to change them, so it really depends on the person. But certainly a positive person can influence a work place a great deal. I know one of our assistants is a very positive person and a lot of the other assistants say, "Well, when I feel bad, I just go and talk to him!" because he will always get you out of it.
It would be nice to be that type of person who does not complain and has a positive view on everything. Yet if that other person doesn't want to get out of it, then you can still be the person like our assistant. So not to fall into it with the complaining person is very important but not to preach either is also important.
It's difficult to accept that sometimes we don't have the power to change things and change others especially.