Hatton Garden is the jewel district, as in the area where dealers
view parcels of gemstones in private offices. If your friend is
interested in beadshops this is not the area.
Bond Street is where the high end jewellery shops are located, same
comment about beads applies.
Covent Garden is the area where there are interesting small craft
shops, and stalls in the market. This is where she could start. If
she takes a trip down the river to Greenwich there is a great market
there too, with many jewellery stalls.