I can imagine arguments that say it would be, but if you can show there was no meeting of the minds (you were joking around) that should get around it.
This is not legal advice and is not intended to create an attorney-client relationship. The post is only an opinion. You should speak to an attorney for further information. The poster is licensed only in CT & NY. Please visit www.hamadlawfirm.com for more information about our services. If this post is useful to you, please remember to upvote it.
No, it is not legally binding. The original lease terms are what governs this situation. You cannot change the terms of the original lease contract without, what we call in the legal world, "consideration." What was the consideration in this addendum (if she wants to call the text message an 'addendum') to the lease agreement? Nothing. As such, this has no legal significance and you cannot be held to it. Simply hold up the lease, show how it says nothing in there regarding laundry being part of the lease, and that's all you'll need. If she attempts to pay you less because she has no access to your laundry facilities ask her where in the lease it says that laundry was part of the rent. She won't be able to, but she'll say "It's part of the lease because of the text message." To which you will reply, "That was simply a nice gesture on my part. And since using my laundry facility was not in the original lease contract AND you have given me nothing additional (i.e., consideration) to use my facilities (i.e., there was no new contract, because a contract cannot exist without consideration by both parties), you are out of luck and owe me the full amount of the rent."
Disclaimer: I am licensed in CA only. This answer should be seen as general information only and not relied upon as legal advice.