I have a mother-in-law's tongue, Sansevieria trifasciata, that's getting a bit yellow and slightly droopy. A friend recommended that I add some coffee grounds to the soil, which I've been doing, but I'm worried that the grounds are making the soil too acidic. Do you have any suggestions?
No; the current fad for coffee grounds use is just that, a fad, and its not helpful for almost all potted plants. Remove the coffee grounds, or as many as you can, they don't do any good at all and are often positively detrimental. This was confirmed by James Wong (a professional and famous British ethnobotanist and garden designer) who once touted coffee grounds as great for everything, having to finally confess very recently that he'd rescinded that advice; he'd put them in his houseplants and they all died. Small quantities are good on the compost heap, maybe a few spread round acid loving plants outdoors if you must, but don't use them on houseplants.
Regarding the Sansevieria (mother in law's tongue) it might be helpful to give more information and add a photo if you can - info such as how long you've had the plant, your watering regime,the level of light it receives and so on, in order to maybe get a diagnosis of what's wrong with it (well, other than the fact its been made worse by coffee grounds!).