How should I proceed if my mother-in-law left no Will and had live-in boyfriend who is not cooperating?

Question Details: She passed away and her live-in boyfriend of 15 years is not cooperating. She passed due to end stage cancer that showed up abruptly and only took 4 months from time of being diagnosed. During this time she spent 3 months living in hospitals, skilled nursing and hospice. During her brief stay back at home, we asked her about her wishes and what the future would look like. She responded letting know her burial wishes and said everything else is already taken care of. We didn't want to push the envelope since she was in constant pain and on many painkillers and high liter oxygen. When she passed, her live in boyfriend became very concerned with his living arrangements and said that everything is his. She didn't have any physical property having leased both her house and truck. After this passing, we found out that her boyfriend went to where they leased the truck and brought back a representative to the ER to have her purchase out the lease with his name on it. We tried to get family heirlooms from the boyfriend and he expressed that they were important to him and he is keeping them. We don't have receipts for old family items and he says she gave everything to him. We just found out that he used her pre-signed checks to pay the rent 4 months in advance 2 weeks after her death and also used her credit card and debit card. He wasn't listed on these accounts and knowingly did so since he doesn't have any money and was taken care of all of the last 15 years. The beneficiaries on this account was my wife and him at 50% apiece but he literally spent it all but $3,400 that was in the account. As for her 401K, he's listed as the beneficiary, 100%. We found if odd that she choose not to set any money aside for her only grandchildren and when you see how the beneficiary is listed it seems like he could have changed it over the internet with her information. Can you change a 401K beneficiary on the banks site? She trusted him to take care of everything during these final 6 weeks and it just seems so strange that he is getting everything and not even talking to us. in fact, he's sold items that were pre relationship for cash during this endeavor. What, if anything, can we do.

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