The story's pretty tight-knit already, so in order to make Hol Horse turning into a good guy work out, his involvement would probably be one of these:
- Since the series has been good with teasing, it would be interesting to see Hol Horse get separated from the group and run into Pucci, who promptly kills him for betraying Dio. While it wouldn't happen onscreen, the assumption would be that this is Dio and Pucci's last day together.
- During Jotaro's fight with Dio, Hol Horse attempts to save Jotaro, similar to how Polnareff does. Except he'd probably die from it.
- Nukesaku kills him because LOL
This is of course ignoring the fact that earlier that day, he shot himself in the face multiple times and was hospitalized.
OF COURSE you could get rid of the whole bit where Hol Horse and Boingo team up (which was a silly arc anyway, and if they cut it out, I wouldn't miss it at all), but as mentioned above, it'd be pretty hard to put him in without it looking ridiculously tacked on.