Hey Pam -
With that one particular client I would raise him/her up a little at a time. I would even tell them, in some very diplomatic way, that they are not currently paying what everyone else is - and make them feel special that you kept them low all those years. In other words, schmooze them. It is not unreasonable to raise our rates every year, or at least 2. He/she has been getting a bargain.

So maybe let them know that the new rate will occur beginning March or April (give them time to work it into their budget) and let them know you'll be doing it in 6 month increments over the next 2 years, but then make the increments match what you'll be raising your other clients to next year.
So...if you were at $10.00 an hour now (totally for example and not at all realistic to those who would think this is)...and you are charging $13.00/hr. for everyone else - and next year you want to increase them to $14.00/hr. then you would increase him/her $1.00/hour every 6 months for 2 years, so by that time everyone is at $14.00/hr.
HTH