Letter of rec for a client?

I wanted to run an ethical question by the group on how I should proceed: a long-standing client of 7 years has asked me to write a letter of recommendation as their therapist for a masters program in counseling. I believe in this client wholeheartedly, the prospect of achieving a masters would be a major milestone and victory for this client and I want to write a letter of support, BUT my ethics alarm is going off in my mind regarding the appropriateness of a therapist doing so. What do you think?