There's no direct interaction between the winny & b/c, however the actions of the two can be a bit in conflict. B/C regulates estrogen and can actually encourage estrogen-pattern fat depositing. While the purpose of the cycle is in general in support of a particular push to build muscle, lean out, etc.? Obviously the type of effort -- bulking or cutting -- will depend primarily on diet. The actual outcome (ability to lose bodyfat during a cutting cycle) may be inhibited by the b/c. This is why generally competitors cut out b/c or use non-hormonal birth control methods like IUDs. But can depend on the person & the b/c.
She will most likely encounter an interruption in her cycle which also sort of conflicts w/ the point of the b/c -- keep you regular. I can't speak to how effective the b/c is then while on the winny. Maybe someone can speak to that. Personally I'd use a back up method of b/c. But the interruption will probably occur w/ the winny cycle. Did she encounter this on the previous cycle? Then she would at least be aware of that. And the additional chance of interruption of menstrual cycle while losing bodyfat below a certain level , e.g. around 12%.
If the cycle is a bulking cycle, then I don't imagine the b/c would really matter as far is inhibiting fat loss. But that's a call she has to make based on goals & response to the b/c.