OK. "le" is the indirect-object pronoun meaning "to, for, at, from him, her, it" The idiom here is "le duele" = is hurtful TO her (Andrea) so the answer is:
El estómago le duele mucho a Andrea.
If you want the Present Perfect, the infinitive is an -er verb so the past participle is "dolido" = El estómago le ha dolido mucho a Andrea, BUT because the question uses the Present Indicative (what you learned first in Spanish I), the answer must be in the same tense.
The verb "doler" (to be hurtful/painful) does not function like most verbs: yo, tú, él/ella/usted, nosotros/nosotras, ellos/ellas/ustedes BUT is used in the 3rd person only usually. IT is hurting (duele) or THEY ARE hurting (duelen)
Examples: Me duele la cabeza = My heard hurts, but literally it says "to me is hurting the head"
Me duelen los pies = My feet hurt, but literally it says "to me are hurting the feet."
Feel free to ask any questions you may have. Other verbs like this are "gustar" or "importar."
Thank you very much! I am still a little confused, but it makes much more sense to me than it did before!