I currently have a group of high schoolers who just don't do the required conditioning at all, and if they do it's a total joke. For some it works to explain to them that they are doing this because it will help them be stronger, able to learn harder skills, and less likely to get injured. If they tell me it's not fun, I inform them that it's a heck of a lot better than an injury. For other kids I will have to stand and watch/monitor them constantly to ensure they get their conditioning done and not let them move on until it's complete, not so much of a treat for teenagers. If it comes to a point where the other kids are ready to move on, move along with them and have her go off to the side where you can still see her until she gets the work done, only giving her as much attention as she needs to do the skills correctly (this is provided she is capable of doing the skills- I wouldn't do this to a kid that tried their best and just couldn't get it).
If you think it's that she doesn't have the strength to get the skills done correctly, just remind all of the kids as a group to try their best and try to improve a little bit each day. Praise the progress they are making along the way. If you're not sure if she's faking it, talk to her parents. They will probably be willing to discuss it with her and find out what the problem is and report back to you. Some kids, especially younger ones, have a hard time controlling their bodies for conditioning and just need to be worked up to class expectations a little bit slower than other kids. In those cases I try not to call them out on it or make it a big deal in front of the other students, but quietly give them permission to do a few less reps as long as they try their hardest, because doing an extra 5 reps of sloppy conditioning just isn't worth it in my mind if they just aren't capable of it yet.