This is not a substance we have experience using in home gardens, so would not encourage its use as you don't necessarily know what microbes you'll be introducing to the soil or the plant's surfaces. (Beneficial fungi we can't see can live atop plant leaves, for example, which may protect them from infection by pathogens.) You also don't know what specific other nutrients or acidity level the whey would expose the plant (or any of the beneficial insects that might visit it when hunting for prey) to so here too, it's probably too great a risk compared to what few benefits it would probably provide.

A Cornell Extension publication discussing whey in commercial agriculture (farms) mentions a drastic pH swing that fluctuates repeatedly as the application degrades and interacts with soil conditions and microbes. (The change of two pH points may appear minor but this really reflects a one-hundred-fold change in acidity level.) This would almost certainly stress live plant roots and beneficial soil microbes or insect soil life and would not be recommended; plus, they state two pH ranges for types of whey, both of which are quite acidic.

Roses prefer a pH of around 5.5 to 6.5 depending on the reference (there are so many hybrids of roses that it likely varies). The only way to know what the current soil conditions are in terms of acidity level and nutrient analysis is to have the soil laboratory tested. (Home test kits and probes tend not to be reliable or accurate enough, nor will they provide as much information as a soil test.) If you think your roses are struggling due to improper soil pH or a nutrient deficiency, the test results can guide you as to how much lime or fertilizer may or may not be needed to adjust it.

Miri