I have done a very similar conversion project for my renegades and heretics using Veer-myn, Victoria Miniatures Beastmen (
https://victoriaminiatures.com/collections/broolian-beastmen) and various
IG parts.
With the regular Veer-myn I learned a few lessons:
-The heads are comically large, even compared to Cadian bubble-head. The models come with a lovely, bulky torso that fits well with the head.
-The torso have bulges on the side to attach arms to, but if you cut/shave these flat they fit
GW human arms perfectly (I used catachan arms).
-Human legs look super-weird on a Veer-myn torso, so I ended up just sticking to an arm-swap and I liked the result.
I built my Ratgyrn pretty much straight out of the box and love them. They scale basically perfectly with the old metal Ogryn. I'd wager to guess converting these with newer plastic Ogryn/Bullgyrn parts would be really easy.
If your friend is just looking for abhumans in general and would like some infantry that matches
GW human scale a little better, Victoria Miniatures beast men are scaled perfectly. You can basically swap their parts freely with most Imperial Guard parts.
If you have any questions or want any more part/model comparison images, let me know. I love working on projects like this.