Should have the plumber put a camera down the line to see if there is a bigger issue, which it seems like there is since it's clogging so often. If there is a bigger issue, can use that to get the landlord to fix it. As far as DIY, running an auger down a public toilet on a weekly basis wouldn't be high on my list of things I want to do. It can be a bit messy and if there is a real problem then the landlord could blame your in laws.
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I second this. I'm guessing there are pipes that need replacing somewhere, but that's impossible to know without looking.
If the blockage is really 100ft down the line, the hand auger would be pretty useless. I've only had limited luck with hand augers... and that's usually been when they're really close to my point of entry. I've never looked into buying a real auger... but I know when I looked at renting them, the rental was approximately equal to what my plumber charged. Having "experienced old codger" running the machine for the same price seemed like a bargain to me.
I will warn you: this was an old-school plumber that was, by comparison to every other plumber I'd met, very inexpensive. Part of this was that he was very experienced and really fast with every job he ever did. Part of it was that he had a real attitude and "couldn't believe those sons of bitches were charging so much for easy jobs." I moved away from that city and I miss the bristly old bastard. In other words: YMMV.