I see no reason why HIPS should not be suitable for ultrasonic welding. In the third link you see a youtube film which shows ultrasonic welding of HIPS. The first and second link shows that even different polymers can be welded by ultrasonic welding, if they are at least semi-compatible. 

https://www.dukane.com/us/Documents/Thermoplastic%20Comp%20Guide.pdf

http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-is-it-possible-to-weld-dissimilar-plastics/

https://www.youtube.com/watch?v=VtaGQX6PTH0

HIPS is compatible to HIPS in vibration welding. Do you know the viscosity of HIPS he is using? As you also know, viscosity can be an important factor that is affecting weld strenght.

Weld time is also important:  "Increases in weld time increase energy dissipation in the materials being welded. If weld times are too long, however, large amounts of melt flow are produced, which orients polymer chains in the flow direction along the weld line and produces large amounts of flash, decreasingthe strength of the weld"

So, if the viscosity is high and the weld time is not proper, these can create a double effect which is decreasing the weld quality.

-What is the viscosity of HIPS he is using?

-What is the viscosity of GPPS he is using? (you said he got good results for GPPS)

-There is no concrete research on the vibration welding of impact modified thermoplastics. So, it can be a valuable study to compare:

--GPPS vs HIPS

--PMMA vs impact modified PMMA

--PA6 vs impact modified PA6

I can give some support for such a study.

https://www.sciencedirect.com/search?qs=Vibration%20welding%20of%20High%20impact%20polystyrene&show=25&sortBy=relevance

https://www.sciencedirect.com/science/article/pii/B9780815515814500056