It's on Allen, Hiller's not going to leave unless he gets a better offer elsewhere, which will obviously never happen or ofc he get's fired, he's not going to fire himself though. Hiller will never be an O-line coach at another P5 school, he has an extensive track record of being an utter disaster at this level. Whenever he leaves Indiana (which should've been less than 24 hours after the Purdue disaster), will be the last time he coaches at the P5 level.
I've had some minor critiques of Allen in the past, but most of them are really not a big deal enough for me to get mad over, this is mind-boggling though, I never thought he would be dumb enough to risk that 5 million dollar salary for one assistant who has completely derailed the entire offensive unit, but here we are. I saw Allen talk about how difficult it was for him to fire Sheridan, and look I understand that's not a fun decision, but you're paid 5 million dollars a year to be able to make the correct decision about assistants, even when it's difficult. I don't have any sympathy for you saying it's difficult to fire assistants after going 2-10, either your assistants are getting fired or you AND your assistants are getting fired because even at Indiana, 2-10 is not going to cut it. If you are that close to your assistants off the field that you can't give them an objective performance review, maybe you're just not cut out to be a HC at a P5 program.
Only time will tell but if he didn't fire Hiller after this past season, I don't think he ever will. Ofc Dolson gets some blame too, he couldn't care less about the football program, better AD's would've stepped in and told Allen that Hiller needed to go, e.g. the Notre Dame AD had to step in and tell Brian Kelly to fire his DC(Brian VanGorder) in 2016 even though BK didn't want to. But Dolson is too caught up in the basketball program to waste any extra energy on football. A lack of commitment to football from the top down is the reason Hiller still has a job at Indiana.