what does "mess up with her"mean?
Eli just grinned after her. “She’s amazing.”
“Yeah, well. Look, Eli, I like you. At least I liked you back in the day, and I’m inclined to like you now. Just don’t mess up with her.”
“I’ll be working hard not to. We might as well go in and sit down.” He turned toward the parlor, stopped when Vinnie studied the massage table. “She won’t take no.”
“Not on much.” Vinnie hooked his thumbs in his uniform belt. “Anyway, Eli, I know Detectives Corbett and Wolfe came to see you.”

Question

what does "mess up with her"mean?
Eli just grinned after her. “She’s amazing.” 
“Yeah, well. Look, Eli, I like you. At least I liked you back in the day, and I’m inclined to like you now. Just don’t mess up with her.” 
“I’ll be working hard not to. We might as well go in and sit down.” He turned toward the parlor, stopped when Vinnie studied the massage table. “She won’t take no.” 
“Not on much.” Vinnie hooked his thumbs in his uniform belt. “Anyway, Eli, I know Detectives Corbett and Wolfe came to see you.”

Answer

I would interpret that as "Don't do anything stupid to ruin your relationship with her."

Answer

When someone messes up it involves making a massive, irreparable mistake e.g. getting drunk and kissing your best friend's girlfriend. It's pretty big. In this context the character is telling the person that they shouldn't make a mistake with the girl who has just left. This would all be much easier to explain if I could use swearing. :)