Veterans Affairs has initiated a program called "Veterans Pride" patterned after the Australian and New Zealand program, where veterans wear their medals in civilian attire on holidays to promote Veteran Pride..
Veterans Pride, VA guidelines

I know these guidelines are specific for Holidays, but it leads precedence to wearing medals in civilian attire.

I believe the subject took a wrong course with the assumption of wearing a kilt as part of the military uniform, when I believe the original poster asked about her father wearing his medals with a kilt.. as directed by the Veteran Affairs, encouraging Veterans to wear their medals in civilian attire, which the kilt is definately part of this catagory.

I was reading in the Marine Corps uniform guidelines and it authorizes former military to wear medals in civilian attire.
USMC REGS clearly state:
a. Men. Miniature medals may be worn on the left lapel of the civilian evening coat on ceremonial occasions, but only when strictly appropriate to the occasion. On the full dress and tuxedo coats, miniatures will be worn centered on the wearer's left lapel, placed horizontally and one inch below the end of the collar gorge. The top edge of the holding bar will completely cover the buttonhole. If current styles preclude wearing on the lapel, miniatures will be placed in the same relative position as if there was a lapel.
b. Women. Miniature medals may be worn on civilian evening attire on ceremonial occasions, but only when strictly appropriate to the occasion. Miniatures will be worn in approximately the same position where they would be worn on the uniform. If current styles preclude the wearing of miniatures in a place approximating the position where they would be worn on the uniform, miniatures will not be worn.
c. If a second row of miniatures is worn, it will be placed one inch below the first row. In no case will more than two rows of miniatures be worn on civilian attire.

Now of course, these are Marine Corps directives, and all services have their own directives, but of course it may be safe to say, if the Marines have guidelines on wearing medals in civilian attire, it could possibly be safe to say the other services have these simular guidelines.