Regarding the Mother ***SPOILERS***
So after watching the movie I just couldn't help by feel that Liza's mother totally screwed Wilson over super bad.
The way I see it is this. Ok so he finds out that his wife left him this letter and he agonizes over it for some time (while huffing gas) and keeps it in the silverware drawer and then when travelling keeps it on his person.
After running around with Denny being crazy and high he comes back home and seems to settle. Gets clean, applies for the job.
It is then that (we don't know it yet) but all of his stuff get stolen, including the letter, at which point he re-lapses hard into the gas huffing, alienating Denny as well as coming very close to trouble with the law (the huffing kids) and loses his prospective job. Later we find out, the mother is the one who stole everything with what seems to be the intention of reading the letter for more insight into why her daughter killed herself but she just didn't have the stones to actually open it.
With the way Denny kept bugging Wilson about it I think Wilson was jsut about at the point of opening the letter while being clean.
So essentially I think the mother screwed Wilson over very badly by stealing his stuff and causing him to relapse just as he was getting back on feet, getting clean, and just about to open the letter.