If she fertilized by burying a fish next to the rose ( like the Pilgrims), she could try something faster acting, like miracle grow, to get a quicker confirmation of her hypothesis ( that it needs fertilization).

I would be very difficult to overwater a knockout rose during the summer months unless they were in a swamp. Dig a bit around the rose and unless there is actual standing water overwatering is not the issue. If she already fertilized with something like Miracle grow like Rifis mentioned just give it a few days to work. If she tried something organic or slow acting then a good dose of Miracle grow can perk them up 😀. I also really like the liquid Bayer fertilizer. Be sure to water in any fertilizer you use. Let us know how it turns up.