she should avoid tdf for 1st trim and can continue from 2nd
Your research is correct. Tenofovir is usually given at the start of third trimester, so you have to ask you doctor why he/she recommends an immediate start. However, many women do have exposure to Tenofovir throughout their pregnancy and generally it is safe. Some head size and bone mineral density issues in the newborn are noted. So it is very much a benefit and risk exercise when deciding to take Tenofovir before the recommended third trimester. There is a recent paper discussing these issues that you may find useful:
Hepatitis B in Pregnancy
Tram T. Tran
Liver Transplantation, Cedars-Sinai Medical Center, and David Geffen School of Medicine at UCLA
http://cid.oxfordjournals.org/content/62/suppl_4/S314.full