I guess u/s aren't 'necessary'. But they can be useful at the 20wk scan to make sure everything is ok. Some things can be picked up at that scan that may need to be dealt with asap when baby is born, so I personally think it's worth having at least that one scan.

Is she not seeing a dr at all? Probably listening to the heartbeat and other 'observations' could be sufficient.

It is true that women have been giving birth for a lot longer than u/s have been around
There was also a much higher mortality rate for both mums and bubs back then. I personally think u/s have a place in pre-natal (is that the right word? I get them all muddled hehe) care. But that's just my humble opinion. heh.