Tricky one mate and only thinking out loud here, objectively, and without my work / ethical head on. I nearly deleted this post, because I thought it sounded a bit egg sucky, but I'm sure you know me well enough that's not the case - it's just rambling
One side of the coin, I guess it could be argued that if
A) The stand in boss is only there for a couple of weeks
B) The Usual boss, who's coming back, wouldn't allow i
C) It's therefore not something that will happen again, no harm's been done, and will probably become dead and buried...
Maybe it would be best to let sleeping dogs lie, as such.
The rationale behind this potential scenario being, will it cause you problems at work, if you have to give evidence at a disciplinary meeting/hearing, potential tensions with colleagues (if they don't get sacked and stay in post) - and the remaining ones thinking you're a "grass" and alienating you - will you then end up with months of the shit jobs - (I don't know what they're like, this is a potential thought based on any workplace and not a slight on them).
I'm thinking back to when I reported something terrible when I worked for an NHS Trust, and as a result, the managers and HR all closed ranks, and my life was made such a misery that I got a new job and left.
On the other side of the coin, what they did is wrong, and whilst unlikely, they could have killed someone on the roads in the worst case, and questions around insurance would arise, and in any case your employer could be subject to potentially huge compensation and negligence claims, and, just as a very worst case example, you could end up explaining to a coronors court why you knew this practice went on and didn't raise concerns..
.
(I'm obviously being very far fetched and extreme, but you get the jist)...
It's actually a really tricky call, and not one I envy you for