I would continue eating Soylent like normal if she feels like it (although maybe she won’t). It’s just food. Probably during the other 1/3 of whatever you eat there will be enough variety to compensate (I was kind of assuming the other 1/3 is a normal varied lunch or dinner meal and not, like, a bowl of Cheerios every day). People eat all kinds of unhealthy things when pregnant, it’s not magic. There are some extra vitamins/minerals recommended to pregnant women as others have noted so if it were me, I would just make sure that with the other 1/3 of varied food plus Soylent plus taking prenatal vitamins these recommendations are met. There are about a thousand other things that have more influence on the outcome of a person’s life and health than what is eaten during pregnancy (assuming there is no medical crisis, like getting listeria or something) so it’s probably better to try not to overthink it.
If you like reading medical studies and relying on scientific evidence as opposed to erring on the side of potentially irrational caution about pregnancy guidelines, I highly recommend the book “Expecting Better” by economist Emily Oster! It’s a great read even if you are just interested in economics or analyses of medical data and not at all in pregnancy.