I say don't do it and try something else, but hey, it's your body, fuck it up if you want.
Ugh, don't listen to that bullshit.
Anyhow, nausea and/or vomiting may occur even with pure powder, but I tend to have mild nausea if any on the powder. Taking it on an empty stomach and with ginger will reduce nausea. As much as I love DXM, it is a bit taxing on the body, so be prepared for a comedown the next day (although you may have an afterglow as well).
Use a dosage calculator to send you where you would like to go. A third or high third plateau experience is comparable to a k-hole, only it lasts me around 3 hours rather than not quite one.
The taste is absolutely rank, so make sure you have capsules. Honestly, it's the worst tasting substance I know.
Since you have done ketamine, you should be prepared for your journey, but I find DXM the darker, deeper cousin. DXM isn't a fun drug for me, not to say it isn't enjoyable, as it is among my favorite drugs of all time. It is just an intense and useful introspective tool. I find it more entheogenic than ketamine, honestly. The doses that your girlfriend are taking are comparable to being stoned and a little drunk at once. Music IS fantastic on DXM, at any dosage range.
The small test dose is a very good idea, because there are some people who are extremely sensitive to DXM because they do not have an enzyme in the liver that metabolizes DXM properly.