Love it - that tank top is ideal! Even better if she would wear it without a bra. Maybe you can get her to try that at home and get her comfortable going braless. Then try somewhere away from home, not just shopping at a local store. Seems to help to be away from usual surroundings and the chance of seeing someone you know.
You have discovered the tricks of adjusting strap lengths and making certain any previous marks in the material are not obvious. A little water and hanging to dry works wonders...