Can mothers bf throw away mail that's not his?

Is the address where the mail is going the same address on her state issued ID? If yes then she legally resides there until she's evicted. If not the legal residents should be returning the mail to the post office so she can't establish residency. Even if she has legally established residency through squatting over the years the BF/OWNER can certainly still evict her if not out right toss her out because she doesn't have a lease. Most jurisdictions extend a 30 day month to month lease if she can prove she has been paying rent for more than a year but this just means she has 30 days to move if evicted. In some jurisdictions she can be evicted in 3-5 days without a lease for any reason. NOTE: An eviction on her record is a big hit on her credit rating it will make it nearly impossible for her to rent anything for 3-7 years.

Knowing her mail is being throw away and proving it are two different things especially if she isn't a legally recognize member of the household. You can't just extent your living rights to any hobo you pass on the streets the people on the lease or the owner of the property are who decides who can get mail at THEIR home. You can certainly complain to the local postmaster but they will probably just warn the property owner to return any future unwanted mail and then your GF will be looking for a place to live.

>can we take him to court?

Sure as long as you are adults and you can prove he tossed the mail in question (minors can't sue their parents have to do it for them). But you'll be waste a filing fee over a check that can be rewritten once it is confirmed it was "lost" in the mail all you have is hearsay and that isn't evidence. BTW when you take someone to court you have to take a random day off work to go to court they don't let you schedule your case. Do you have a new place to live lined up because I'd EVICT you the moment I got a summons.

>he has been throwing away her mail for years now but it was never important.

Of course not because if she made a fuss he would have tossed her out just like he's going to do if you start trouble over this lost check. Your best bet is for her to get her mail sent to a PO box so it can't be tampered with if her check doesn't show up she just asks for it to be reissued to her new address.

Ever heard of a P.O. Box

No mail tampering is illegal.

That would be tampering with mails - a Federal offense. Report it to your local Post Office.

She doesn't live there, so yes. There's an easy way to stop that. Get a PO box or have her use your address. Since she knows she may be missing important things, she should have done it long ago.

This is called tampering with the mail, report the incident to the postal authorities.