If I was a betting man...
I am guessing that his sister's multiple attempts to speak with her former lover are more than being infatuated...a pregnancy perhaps? Also..the warning signs of a possible self harm attempt are floating around...
A bad reason to fire someone but I’m under the impression Peter is not really a good employee, even for a job as simple as video rental cashier? From what we have seen: he shows up hungover or drunk, he forgets to charge for rentals, endlessly lets friends and friends of friends mooch on his employee account, is rude and/or ignores customers, and at least once he has been suspended by Greg for having his head so much into his own problems his boss ends up doing most of his job.
Everyone has lost Wylie and the band.
Peter has lost his roommate and friend, Patrick for at least a long time in a very bad way.
His friend, Christo, came between his friends Andy and Melanie, not that it was entirely his fault; he bore plenty of blame.
Also Michelle is gone, for reasons unclear and painful.
Now Peter's job and childhood friend implode on him.
Did I forget anything? Probably.