Would my daughter be at high risk for infection if she get her belly button pierced, since she had her spleen removed?

Yes, higher risk: Having a splenectomy may put her at higher risk for a skin infection depending on a variety of factors. You should consult directly with her pediatrician and ideally with a pediatric immunologist.

Depends on situation: If the process is done in a proper way with good preparation and sterile technique I don't see a problem. If she's planning to do it at home or with her buddies at a kiosk by a tech with 2 hours of training I would be worried.