Hi lakookoo!

I can't think of any medications that would prevent someone from following a low carb plan. Your relative may need some adjustment to her dosages, particularly her blood pressure medication, once she has been low carbing for a while and some medications make it difficult to lose weight on a low carb (or any other for that matter) plan.
It's anyone's guess what her primary care physician will think of low carb and whether or not he would encourage or discourage her from trying it. For that matter, Atkins isn't the only plan out there; she may benefit from comparing a few different plans such as Protein Power, The Schwarzbein Principle or others to see which would work best with her own personal tastes and lifestyle and her own physician may be more open to her trying one low carb plan over another. Either way, it's a good idea for her to get checked out before she starts and hopefully get her doctor's support. If not, cutting her carbs to maintainance levels would certainly be a step in the right direction and may make it easier for her to adjust her carb levels lower if and when she decides to do so.