Aslan harnesses are great. They hold well and are comfortable. If she has sensitive skin she might be a bit irritated by the stitching, which (as I understand it) the SpareParts harnesses don't have.
Sportsheets makes some good inexpensive harnesses. Bare As You Dare is OK for giving a bit more hold on a "strapless" double, but it's a little thin for an ordinary dildo.
Aslan Leather harnesses are excellent, as are Outlaw Leather and SpareParts - but you pay for the excellence.
All of the ones I've listed can take a double-ended dildo, and some can be set up to use two single-ended dildos to give her more fun.