Pay and compensation query

Someone who reports to me has been in their job for 6 years having had a promotion once in that time. She is now doing the same job as someone else who has worked for the company for 12 years.

The person who has been working at the company for 6 years is miffed she is being paid 13% less than her colleague who has been there for 12 years.

I have said that apart from the fact I don't set the pay rules in our company, someone who had been in service longer is more likely to be paid more as they have benefited from pay increases in the past. This didn't cut any mustard and she is well pissed off, partly with me and partly with the company in general.

Any advice? This person feels they should be given a bump-up in pay, which is something my company offers in exceptional cases.