I would definitely ask them if they are alright - I'm not the best with crying people, but the least I could do would be to offer them a tissue, the use of a telephone, directions, or the simple courtesy of acknowledging that there is a person suffering in plain sight. If they don't want my help, then that's fine and I wouldn't take it personally. There are plenty of times when I've been upset, even weeping in public, and for someone to just talk to me would have been so incredibly grounding and distracting.