It looks like she has a fungal infection. Get a topical anti-fungal cream from a pharmacy (active ingredient either clotrimazole or miconazole), apply it sparingly with a q-tip and rub it in very well twice a day. You want to rub it in well so she can't rub any off with her feet and then eat it as these medications can be dangerous if ingested. Repeat this until several days after it looks healed to make sure the fungal infection is gone. Make sure to wash your hands after handling her as fungal infections can be contagious to humans. Watch your other guinea pig for similar patches, but do not separate them as the other has already been thoroughly exposed and separation will just be stressful for them.
If the cream doesn't seem to be helping after about a week of treating post back here and we can suggest other treatments to try.