I'm on the 18th floor and have grown as low-growers/mounders/trailing plants (non-vines):

petunias
alyssum
sweet potato vine (which grows more as a trailing plant)
wax begonias
impatiens
vinca vine (which will grow as a trailing plant)
verbena

Probably others over the years...

I think if they are concerned about the plant getting whipped around, they can look for creepers and/or ground cover types, trailers to go around the edge, and low mounding plants (where those would get continually pruned to keep them bushy and low). Something like an ornamental grass, sedge, or the dracena "spike", would give some tall interest in the center of the urn but would be better able to handle the wind.

How much sun do you have? AM or PM? My geraniums are pretty hearty and I'm 18 floors up. I'm S/SW. We get all sorts of abuse here in the city. All sorts of coleus do well; my hibuscius thrives. My canna start to look a little ratty after the September winds though. I tend to do canna and coleus together. I love the tall large leaves and the vines hanging down look dramatic. I also did a bunch of lemon balm/citronella this year. Great leafy green plant - grows like a weed. My coleus flowered this year too which was extra nice.

Avoid petunias unless your elderly relatives want to dead head.