Yellow leaves are often a sign of stress and could be from watering too much or too little. In this case I would guess too little but don't know enough about the size of plant and how sunny, windy or hot it has been. I usually have to water the ones in that size once a day or every other day this time of year. Hibiscus need to be moist but can't stand wet feet or bone dry. Also they are gross feeders and need regular fertilizing to do their best. A 3-1-2 is generally workable and some additions of epsom salt is good (magnesium sulphate).
Hope this helps give you some ideas. I usually don't keep hibiscus in 1 gallon size more than a month or so after buying unless they are small for this container size.
As to the coffee grounds they do prefer slighly acidic conditions but I wouldn't use too much or too often. Just a few spoons full every other month maybe to supplement your regular fertilizing regime.
Happy Growing David