How do you know when someone who's depressed is using it as a crutch?

I know someone who has been diagnosed in the past with being depressed. I respect and understand that. But this person also seems to be picking their spots when it affects them most. I feel bad saying that but there's no other way to explain some of the behavior. They act like they're too "down" sometimes to go out, to help out with a project, to go to the grocery store and this particular person rarely ever does any house work. Often times, I'll come home and they will have just been sitting around, basically doing nothing all day. However, when it comes to going shopping, on vacations, seeing certain friends and relatives, they pretty much always feel up to it. The way I understand depression is that it's a disease but it seems like other things will affect this person's behavior or they're using it as an excuse not to be responsible, do things they may not want to do, be taken care of as if they were a child, etc. I do not feel good about having these thoughts because I want to be sensitive to their needs and their disease but I also cannot help but have this frustrated and cynical feeling about the times they seem to have more issues. Thanks for any help.