Assuming you are referring to genital herpes, they're essentially right. I can't say it's impossible, but it's really, really rare that it happens.
There is something called site preference with herpes that helps with this. Also, herpes rarely goes from the genitals to the mouth like it does from the mouth to the genitals.
Herpes type 2 really prefers the genital area, so oral hsv2 is rare. When it does happen, it rarely recurs (most people with ohsv2 only get the initial outbreak and no others), ir rarely sheds, and therefore rarely, if ever, transmits.
If someone has genital hsv1, it's already not in it's preferred site, so it isn't shedding a lot, recurring a lot, meaning it rarely, if ever, transmits without symptoms.
If someone has ghsv2, it's quite possible to transmit to someone else during genital to genital or genital to anal sex without symptoms being present because of site preference, but not oral sex.
So yes, it's possible, but very, very unlikely. I hope that helps, but let us know if you have any other questions.