I studied tai chi briefly a number of years ago, in Japan with the Navy before I found Aikido.
Last summer I started up again after 6 years of Aikido.
I do agree that it is good to have some mastery of one art before beginning another, but tai chi and aikido use many of the same principles: moving from the center, attention to energy flow, etc.
If the tai chi is mostly about the form, I would say go for it, but it would probably be more confusing if he/she is primarily teaching the martial applications of tai chi.
My instructor has been studying tai chi for 11 years, and is also a chi kung instructor. While his teacher is working on many of the martial applications with him, we are primarily learning the form. He does, on occasion show application to one or two of us with a martial arts background.
His meticulous teaching style has greatly helped my grounding in aikido, as well as improving how well I move from my center.
Just my two cents.