I would tell her and expect the worst and be pleased with the best. From what you have indicated she seems to be "serially discontent" and dissatisfied. She needs something you can't give her.

Do you really want that type of energy/emotion in your life? Because of your history, she may want to just partner up with you again - and are you really wanting that?

One of the greatest liberties we have under God is the pursuit of happiness - which includes the ability to be able to pick those people that make us FEEL good and are good to be around.

However, only you know her - was she negative before or do you think she is in a "slump" right now and just needs some encouragement?

In order to really get the best out of your exercise (remember this is for you, too) your gym buddy has to be able to motivate and inspire you by sharing similiar interests, feelings and emotions and not be a drag in your sails.