Ahoy_NATO said:
The thing Levy didn't want Gajeel to do is fight Bradmad in his Third Seal mode because in the Tartaros arc, Gajeel almost died to the Tartaros curse powers which is what Bradman was using which worried Levy. I do believe you may be correct that it isn't good for him, though it hasn't been stated how long it will take til it has an effect, it seems like a long time though.
That wouldn´t make sense in accordance to the pictures.
She starts denying his decisions once he starts breathing in the particles, before that she doesn´t mention her worry although Gajeel was fighting him with regular abilities.
Beforehand she tried to win the fight herself and protect him, something that´s befitting of her character and experiences since she felt useless during the Tenru Island arc where he took the hits for her and actually every other instance where those two fought next to each other. She´s the brain and Gajeel is the muscle. He saves her during fights and she shines during intelligence missions. Those are their established strengths and purposes.
Back to the chapter,inbetween denying his decision and trying to help him, she already had come to terms with her emotional irrationality and was pitying herself for being a weak fighter. This immediately changes once he starts breathing the particles. Her selfpity changes immediately into worry and just the momment when he´s done absorbing them, she literary utters the line "There´s no returning from this".
I just don´t see how this line could be connected in any sense towards her worry about him fighting an enemie that utilizes curses, when she herself fought a guy that utilizes curses and he also fought him just before without her worrying.
It makes no directive sense to put it in this part of the chapter if Mashima wanted to adress her worry about him fighting someone that uses curses.
Also the proper sentence to express this worry would be get outa here, or don´t fight him, not "there´s no going back".