Hello ma'am and welcome.
Based on the provided information, it seems as though this could be due to a
cyst called Bartholin's cyst. All she needs is the following:
+Warm comprises either directly applied or indirectly (over clothes) several times a day
+Lukewarm water sitz bath with epsom salt as many times a day as possible
+Do not burst it as it can get infected
+If at all it is infected, antibiotic coverage will be required
+Warm water showers 2-3 times a day
+Wear loose and comfortable inner wear
+Change inner wear regularly
+Avoid sexual contact or masturbation
+Visit a doctor if at all it does not reduce or subside within 3-7 days
Hope this helps.
Best wishes.