The great thing about dresses like that [and other tops of similar design] is you can adjust / lengthen the shoulder straps so the dress hangs lower when she next puts it on, which lends itself to better / longer views of her charms. Do not over do it or she is more likely to notice, but if done subltly [maybe 1" or 2 cm] you [and others!] will be rewarded.
Have done this on a number of the wife's dresses always to good effect.
So far she has not noticed - or maybe she has and likes the new look... which is even better!