Do you think it is wrong to suggest to my colleague that she could spend less money on her lunch?
Why would you tell her to spend less money on lunch?
She has a periodic expense of 1200USD a month on her lunch
You shouldn't tell co-workers how to spend their money. They may feel like you're overstepping boundaries and judging them. They become upset and cause issues between you two at work.
I don't care what she thinks and I can't stand it when people overspend on lunch.
I can understand how you feel she might be wasting her money, but how someone spends it is up to them solely. Now you can suggest in a friendly manner how they could be saving money on lunch elsewhere, but don't push the issue if it is not of their concern.