I am 22 and have had TMJ problems for as long as I can remember. I recently had a arthrocentesis (lysis and lavage). It revealed that my joint looks like that of a 60-yr old's.
There is really no sure way to tell how bad a TMJ joint is until a surgeon can go in and look at it. All of my xrays from the orthodontist showed nothing wrong. My MRI only showed displacement, when the disc thurned out to be split in two.
If it hurts her, I would not pressure her in any way.I would actually be the one to suggest alternatives for her.
I also like to do it for my boyfriend. Since we are not married, we do not have sex and find that to be the best alternative for him. Now, I can't even fit a sandwich in my mouth, or pleasure him in that way.
That joint is used so much and has so much pressure put on it daily, don't add to it. Even if she loves to do it for you, her health should be the most important thing.