Traditionally, linggam massages are just you being massaged with hands. So long as her mouth or genitals don't come into contact with your mouth or genitals, there is no risk for any STI.
Herpes on the hands is only infectious when someone has an outbreak, which would be visible to you. The skin on the hands doesn't shed - only mucous membrane skin sheds the virus.
I'm not sure what the "adult bath" entails, so I can't comment much on that, but again, if there isn't oral to genital, genital to genital or genital to anal contact, there's no risk.
I don't know where you live, but there isn't a country that doesn't have a fairly high prevalence of hsv1 that I've heard of. Some are higher than others, for sure, but it's not rare anywhere -
https://www.who.int/news/item/28-10-2015-globally-an-estimated-two-thirds-of-the-population-under-50-are-infected-with-herpes-simplex-virus-type-1
In any case, it sounds like it should be fine.
Edit: I do know HIV, Hep B and Hep C are viruses just constructed the sentence wrong. As for Herpes despite HSV-2 being the more prevalent strain when it comes to the genitals, I am equally concerned about the likelihood of the therapist passing on HSV-1. Unlike in the US, HSV-1 is rare in people from our country.