My first instinct is that it's purely a business relationship. Many times two people will establish a corporation for legal reasons. The fact that your T and Mary Smith give workshops together just illustrates the business aspect of the two of them working as colleagues to contribute to the community while building their client base.

Mary Smith probably uses your T's address for business related items to keep things simple and all in one place.

If you really want to know if your T is married, have you thought about asking her? Perhaps can you spot a ring?