I guess alot depends on the skill you're not doing and how close the next meet season is---and your gym's philosophy. Our compulsory girls compete from Sept to Dec and then from Jan to May, they work new skills for the next level or so. By mid to late May, the coaches will make a decision on who to move up based on new skills mastered, work ethic and attitude. Those that may be missing a couple of skills(still need a spot etc) are usually given another month to work on them and then may move up. I know to move to Level 6, they had to have baby giants, clear hips, BWO and BHS on beam and all the new tumbling for the Level 6 routine. June through August are spent doing alot of strength, cardio and learning the new routines. Girls are not moved up during the actual meet season.
A back walkover is such a fundamental skill, that it is hard for coaches to move up a student who does not have it. Not only because it is required for competition but because without the flexibility for a good back walkover other skills will also suffer (ex. Back Handsprings). There was a thread on back and shoulder flexibility, they will help your friend if she works the flexibility at home. Coaches notice when students are doing work at home to improve, and they really appreciate it.