If you buy from private party, it's the seller's responsibility to deal with the tax, you just have to pay the price that is agreed upon.
If you buy it from dealer, I think (don't quote me) you have to tax, because the dealer is an legal establishment. You can call just them asking for it.
She probably paid a one lump sum which included the tax. As long as you two agree on one price then you just have to pay that amount, and he/she will have to deal with the tax.